Arlen Specter went through the motions with his Senate hearing into "Able Danger" after the Pentagon pulled the plug on several key witnesses.
My quick version - there was a data-mining program that produced useful intel, and may (or may not) have explicitly identified Mohamed Atta prior to 9/11.
It was allowed to lapse per plan in 2001, with a major clean-up due to domestic spying concerns, however misplaced. (Lots of this comes from the Pentagon press briefing).
My guess is that a similar program has since been reconstituted, but no one wants to tell Weldon because the intel community thinks he is daft.
Way back when, on August 10, I had nominated candidates for a cover-up:
The obvious whipping boys of summer would be Clinton/Berger/Gorelick, for fostering an overly legalistic approach to the War on Terror; Bush/Tenet/Rice for failing to draw this information from the system in the summer of 2001, when we were at a high terrorist threat level and Tenet's hair was on fire; and Gen. Hugh Shelton and the Pentagon. Gen. Shelton (ret.) was Chairman of the JCS in Jan 2001 - if he heard and downplayed the Able Danger briefing then, he and the Pentagon might prefer to see it buried now.
Like the Yankees, Shelton and the Pentagon have moved to the fore.
(Side note - folks who only recently discovered that Shelton was briefed on this have not been keeping up - Weldon mentioned it in his May 2002 floor speech, and I have been muttering about it intermittently since Aug 10. Weldon recently shocked me by expressing his surprise at this revelation at this press conference, so count him among those not keeping up.)