That fascinates me. At the time I was a manager for United Airlines in Kansas City, and was the Lead Ground Security Coordinator. Prior to Y2K, the millenium, the Clinton Administration sent daily bulletins, sometimes multiples, about credible domestic threats and threats to US citizens traveling abroad. The pervasive bulletins actually caused me to change my honeymoon from Italy to Maui.
Prior to 9/11 there was NO increased security at airports, no bulletins, no advisories. Nada. And we know the Bush Administration advised the Cabinet and Joint Chiefs not to fly domestic airlines about two weeks prior. So they knew of the threat, and didn't share it with the airlines - who, at that time (prior to the TSA) managed the security check points in airports!