It's a conspiracy myth that Powers changed his statements at the request of the FBI. O'Donnell probably. But Powers no. I compiled quote after quote on this matter on my website.
From patspeer.com, chapter 5b.
Kenneth O’Donnell, a Kennedy assistant, rode in the back-up car in the middle seat behind the driver. (5-4-64, 6-4-64, 8-6-64, and 11-23-64 interviews with William Manchester, as represented in The Death of a President, 1967) (Manchester's narration for the aftermath of the shooting) "In the jumps seats, Ken O'Donnell and Dave Powers have heard the sickening impact of the fatal bullet, and Dave has seen it. O'Donnell crosses himself. Powers whispers 'Jesus, Mary, and Joseph...'" (5-4-64, 6-4-64, 8-6-64, and 11-23-64 interviews with William Manchester, regarding the possibility Kennedy was killed by Texas oilmen, as represented in The Death of Lancer, the original draft of The Death of a President, as quoted in an article by Edward Jay Epstein in the July 1967 issue of Commentary Magazine) "They did it. I always knew they'd do it. You couldn't expect anything else from them. They finally made it." (5-18-64 testimony before the Warren Commission, 7H440-457) “We turned—I remember the overpass. And then the shots occurred--which, at that time, I did not know were shots. My first impression was it was a firecracker. And then either somebody said “He has been hit,” or I noticed the slump—he had been waving out the right side of the car and I noticed him slump over toward Mrs. Kennedy, and I realized then that they had been shots. But as fast as that realization occurred, I saw the third shot hit.” (When asked how close the back-up car was to the limousine) “My guess would be 5 to 8 feet…I would presume they were just about turning to step up the speed a little bit, because there would be no crowds from there. (When asked if the Secret Service car had completed its turn onto Elm Street) “My recollection is they had, just about. I don’t recollect a separation of this nature. It was a slight sloping turn, as I remember, and I thought we were right together.” (When asked what Kennedy was doing with his hands prior to the time of the shooting) “He was waving. We had just left the mass of crowds. But as we turned on the grass plot, there were four or five people there, and I believe he waved to them.” (When asked how many shots he heard) “Three” (When asked the time span of the shots) “I would say 5-6 seconds.” (When asked if the shots came in a pattern) “Yes. The first 2 came almost simultaneously, came one right after the other. There was a slight hesitation, then the third one.” (Asked his reaction) “My reaction is in part a reconstruction and is that they came from the right rear. That would be my best judgment.” (When asked how others reacted) “The agents all turned to the rear…I would think watching the President when the shot—the first shots hit—that it would be automatic it would have to have come from the rear. (When asked again about the agents’ reactions) “The reaction I note would be right rear. And again, looking at the manner of the President’s movement I would think you would have to feel the thrust of the shot was from the right rear…He was leaning out waving. He may have just been withdrawing his hand. And the shot hit him, and threw him to the left. He slumped on Mrs. Kennedy. (When asked which shot this was) “It was not the third shot. Whether it was the first or second, I would not know…If I had to pick one of the two, I think it might have been the second shot.” (A 1968 conversation with Congressman Tip O'Neill, as recounted in O’Neill’s autobiography Man of the House, 1987) “I was surprised to hear O’Donnell say that he was sure he had heard two shots that came from behind the fence. 'That's not what you told the Warren Commission,' I said. 'You're right,' he replied. “I told the FBI what I had heard, but they said it couldn't have happened that way and that I must have been imagining things. So I testified the way they wanted me to. I just didn't want to stir up any more pain and trouble for the family…The family--everybody wanted this thing behind them.” (Johnny We Hardly Knew Ye, co-written with Dave Powers, published 1972) "I had just finished speaking when we heard shots, two close together and then a third one. There must have been an interval of at least five seconds before the third and last shot because, after the second shot, Dave said to me, "Kenny, I think the President's been shot." I made a quick sign of the cross and said "What makes you think that?" "Look at him!" Dave said. "He was over on the right, with his arm stretched out. Now he's slumped over toward Jackie, holding his throat." While we both stared at the President, the third shot took the side of his head off. We saw pieces of bone and brain tissue and bits of his reddish hair flying through the air...I said to Dave, "He's dead." (6-15-75 article in the Chicago Tribune. This article reported that a source within the CIA had told the Church Committee that Kennedy aides Kenneth O'Donnell and David Powers had been pressured by the FBI into leaving their suspicions that shots came from the front out of their statements. It also quoted O'Donnell's response to this allegation.) "The story is an absolute lie," O'Donnell declared in a phone interview. "I'm not accusing the reporter, but whoever gave that story is lying. It's an absolute, outright lie." (Later in the article) "I spent four hours before the commission and my testimony is quite clear," O'Donnell said in the phone interview. "I told them exactly what I saw. I was in charge of the whole operation so I know what happened. I arranged the whole trip..."I testified under oath and I stand by it." O'Donnell recalled he told the Warren Commission he heard two shots, the first of which he initially thought was a firecracker. Both came from behind, he said. And Powers, O'Donnell said, recalled hearing three shots, all from the same direction. He denied that either he or Powers ever had suspicions that the shots came from anywhere but the depository. Further, O'Donnell asserted he was never pressured or asked to change or omit anything from his testimony, either by the FBI or CIA. "I met with them every day (while working for President Johnson on the investigation)" O'Donnell said. "Not one of them ever even raised the question.They worked for me. I didn't work for them." (Interview with O'Donnell's son, Kenneth O'Donnell, Jr. by David Talbot, as reported in Brothers, published 2007) (On the source of the shots heard by his father) "He said there was fire from two different directions." (Quoting his father on his father's impressions of the Warren Commission) "I'll tell you this right now, they didn't want to know"...(It was) "the most pointless investigation I've ever seen."
Analysis: from his jumping to the third shot in his testimony, it seems likely that O’Donnell decided that the “firecracker” he heard was in fact two separate shots. His subsequent statements that the first two shots rang out "simultaneously" and "one after another," and that there was a space of five seconds before the head shot, confirm this suspicion. That the quickness of these first two shots troubled O'Donnell, furthermore, is suggested by his subsequent recollection that he'd only testified to hearing two shots.His testimony that he thought Kennedy may have been hit by the second shot--a shot fired only a split second after the first shot, mind you--is therefore of little help to the LPM scenario. It is, in fact, an argument against it. More concretely, O'Donnell's recollection that Kennedy was waving to a small group of people at the time he was hit, and that this happened near a "grass plot," suggests the first shot was heard around frame 190. Since Powers later confirmed O'Neill's recollection about O'Donnell's impression of the source of the shots, moreover, we should suspect O'Donnell's denial of this to the Chicago Tribune in 1975 was, in fact, a lie. His misrepresentation of Powers' impression of the shots--that they all came from behind, when Powers from the earliest claimed he'd had an impression the final shot came from the front--suggests, unfortunately, that he was not above such behavior. First shot hit 190. First two shots may have been bunched.
David Powers, another Kennedy assistant, rode in the middle seat to the right of O’Donnell. (4-8-64, 8-10-64, 10-21-64, 3-17-65, and 5-24-65 interviews with William Manchester, as reported in the TV documentary "The Kennedy Assassination: 24 Hours After," 2009) "I am looking at the Presidential car. His hand was waving and now he put his hands slowly to his throat and slumps towards Jackie. And I say to Kenny 'I think the President's been hit.' Kenny and I not only saw the next one we heard it. We just saw that handsome head get blown off. We heard the shot and we heard the impact of the shot. It was the most sickening thing--like a grapefruit being thrown against a brick wall...At Parkland, I ran up to the Presidential car. His eyes were open. I opened the door and said 'Oh, my God, Mr. President!' I almost expected him to say 'I'm alright' because he never complained. A fragment of the bullet had come out of his forehead. I still get an ache in my head like a toothache where he was hit. I suppose it's just nerves." (4-8-64, 8-10-64, 10-21-64, 3-17-65, and 5-24-65 interviews with William Manchester, as represented in The Death of a President, 1967) (On his response to the first shot) "Powers, in Halfback's right-hand jump seat, shouted at O'Donnell, 'I think the President's been hit!'" (Manchester's narration for the aftermath of the shooting) "In the jumps seats, Ken O'Donnell and Dave Powers have heard the sickening impact of the fatal bullet, and Dave has seen it. O'Donnell crosses himself. Powers whispers 'Jesus, Mary, and Joseph...'" (On whether or not Rufus Youngblood actually climbed into the back seat of LBJ's car, or simply turned around, as purported by Senator Ralph Yarborough) "Dave Powers, who glanced back, confirms the Senator." (5-18-64 affidavit, 7H472-474): “the first shot went off and it sounded to me as if it were a firecracker. I noticed then that the President moved quite far to his left after the shot from the extreme right hand side where he had been sitting. There was a second shot and Governor Connally disappeared from sight and then there was a third shot which took off the top of the President’s head and had the sickening sound of a grapefruit splattering against a wall…My first impression was that the shots came from the right and overhead, but I also had a fleeting impression that the noise appeared to come from the front in the area of the triple overpass.” (A 1968 conversation between Ken O'Donnell and Tip O’Neill recounted in O'Neill's memoir Man of the House, 1987) “I was surprised to hear O’Donnell say that he was sure he had heard two shots that came from behind the fence. 'That's not what you told the Warren Commission,' I said. 'You're right,' he replied. 'I told the FBI what I had heard, but they said it couldn't have happened that way and that I must have been imagining things. So I testified the way they wanted me to. I just didn't want to stir up any more pain and trouble for the family…The family--everybody wanted this thing behind them.' Dave Powers was with us at dinner that night, and his recollection of the shots was the same as O’Donnell’s. Kenny O'Donnell is no longer alive, but during the writing of this book I checked with Dave Powers. As they say in the news business, he stands by his story.” (5-13-76 interview on WGBH TV, as quoted in L.A. Free Press Special Report Number 1: JFK Murder Solved, published 1976) "If the bullet that wounded the President was not the same bullet that wounded John Connally, and I testified that it wasn't, and John Connally testified that it wasn't, then there would have had to be more than one assassin."
(11-19-78 UPI article found in the Reading Eagle) "'I was in the Secret Service car,' said Powers, 'Me and Kenny O'Donnell. When I saw the first bullet hit him as he was waving, I turned to Kenny and said 'My God, they've shot our president.' Kenny blessed himself. Then I saw the second bullet hit the back of his head...' and the voice trails off into silence. Then, very softly, Powers adds, 'Every day I think about it. Every day I get a pain in the back of my head where I saw the president get hit.'" (A 1980 conversation with Gary Mack, as recounted in a series of emails from Mack to John McAdams, posted online by John McAdams, 4-9-03) "Powers told me he and O'Donnell both thought one of the shots might have come from the front. When they told the FBI, the agents didn't take them seriously. Dave was quite insistent on that." (In a follow-up email posted by McAdams at the same time, Mack clarified) "Powers may have told me one or two of the shots might have come from the front--my note to you was not taken from any notes I took at the time. This was a long conversation we had by phone around 1980. Powers told me they didn't know that shots came from the front, just that they thought one or two might have. He never said or hinted they were intimidated to change their story or to keep quiet. But they were disappointed that no one they told the story to seemed very interested in what they thought." (11-20-83 article by Thomas Farragher on Powers found in the New London, Connecticut Day) "The time the first shot was fired, I was 7 yards away from the President. I'm looking at the President. The Secret Service are trained to look elsewhere. And he had been waving to the people on the right side. His hand was way over. And I saw him bring his hand in and then fall toward Jackie. Now a bullet travels faster than the speed of sound. So I saw this happening and then I heard that noise at the same time that I would have thought was a firecracker. But I didn't see the President react that way and I turned to Ken O'Donell (another JFK aide). He's in the jump seat beside me. And I said 'Ken, our President has been shot.' And I remember Kenny made the sign of the cross. I believe that the second shot hit John Connally, and then while we're riding, we're praying. 'You see it's happening behind the agent driving the car--Bill Greer. Great guy. Loved the President. And we're doing about 12 mph but it's happening behind him and he's not aware of it. It seemed to me it was about five seconds from the shot that wounded the President and the one that killed him.'"
(5-30-87 AP article featuring an interview with Powers found in The Evening News) "On November 22, 1963, Powers was in the car directly behind Kennedy's when he heard two shots ring out in succession and saw the President slump down. Then, a few moments later, a third shot ripped open the President's head." (8-31-87 AP article by Christopher Callahan on Tip O'Neill's just published claims about O'Donnell and Powers, found in the New London, Connecticut paper The Day) "Powers, in a telephone interview last week, said O'Neill's version is incorrect. Powers, curator of the JFK Library in Boston, said he did not want to address O'Neill's points directly. 'It's too painful to talk about,' said Powers." (Interview in 1988 TV documentary JFK: The Day The Nation Cried) "Coming down from that short flight from Fort Worth to Dallas, I'm talking to the President and Jackie in the back of the plane and I said 'Mr. President, you wave to the Texans on the right, and Jackie'll wave to the ones on the left.' And this is exactly what's happening when the first shot was fired... I had heard the noise. I'm looking at the President at the same time, and he had pulled his hand up toward his throat and he fell over toward Jackie. There's a second shot, and now Governor Connally is out of sight. The first two sort of came close together, but now we're riding and praying. And now we see the shot that hit the President in the head." (Interview with Charles Kuralt broadcast on CBS, November 1988) (When asked if President Kennedy would still be alive if Bill Greer put the limo's pedal to the floor after the first two shots) "Yes, the President would be alive today, and he would be 71 years old, and he'd be a director here" (meaning the JFK Library). (6-5-91 interview with Lamar Waldron and Thom Hartmann, as recounted in Ultimate Sacrifice, 2005) "We were shocked when Dave Powers, head of the John F. Kennedy Presidential Library in Boston and a close aide to JFK, vividly described seeing the shots from the 'grassy knoll.' Powers said he and fellow JFK aide Kenneth O'Donnell clearly saw the shots, since they were in the limo right behind JFK. Powers said they felt they were 'riding into an ambush'-- explaining for the first time why the driver of JFK's limo slowed after the first shot. Powers also described how he was pressured to change his story for the Warren Commission." (11-7-91 article in the L.A. Times) "I heard the first shot, positively above and behind me," says Powers, who was riding behind Kennedy's car. 'I'm looking at the President like I always did, and I saw him bring his hand in and kind of fall toward Jackie. I said 'Our President's been shot!' and now I see that terrible thing that hit the President on the head, and you never talk about it,' he says, his voice tightening. 'And now the car begins to accelerate.'" (Interview broadcast in CBS program Who Killed JFK: the Final Chapter?, 11-19-93) "I looked at my watch and it was exactly 12:30 Texas time... And then I heard the first shot... I'm looking right at the President and he had his right hand out waving to the people, and now he had pulled it in and it's up around his neck, and he had fallen toward Jackie. And I said "I think our President's been shot."
Analysis: as the leftward shift of the President noted by Powers as a response to the first shot occurred just after frame 190, it is clear he felt the first shot occurred at this time and that it struck Kennedy. His appraisal of the second shot is far less clear. While he indicated this shot came shortly after the first, he also claimed the gap between the first and third shots was but five seconds and that he'd talked to O'Donnell just after the first shot. O'Donnell, as we've seen, heard no shots between Powers' comments on the first "firecracker" sound, and the head shot. This suggests that Powers was talking to O'Donnell when he thinks the second shot was fired, and that he didn't actually hear this shot. His statements to Manchester certainly suggest as much, for there he described but two bursts of gunfire. Powers' associating the second shot with Connally's disappearing from sight, which did not occur till just before the head shot, moreover, suggests that he wasn't sure when he heard a third shot, and only tried to make sense of it later. Since Powers associated the second shot with an occurrence just before the head shot, moreover, he may also have heard the last two shots bunched together, and then moved the second shot closer to the first so he could correlate his recollections with O'Donnell's. Although O'Donnell clearly lied about his own impression of the shots, that Powers' original statement suggests there may have been a shot from the front, suggests that neither of them were actually pressured to change their impression. It seems likely then that O'Donnell changed his story on his own, for reasons all his own. That Powers told Waldron he'd been pressured into changing his story as well, however--when his story doesn't appear to have actually been changed--outside his addition of a shot that hit Connally--is indeed a bit curious. Perhaps he'd said they were uninterested in what he had to say, and Waldron had misinterpreted or misrepresented his words. Or perhaps Powers was simply exaggerating. First shot hit 190-224. Possibly heard but two shots. Last two shots possibly bunched together.