The stories of Able Danger can be read at FR Keyword Search = Able Danger
http://www.freerepublic.com/focus/keyword?k=abledanger
In a nutshell, our intell knew about the terror cell and wasn't allowed to act on it before 9/11 occurred.
No, the complaint is that Able Danger did not miss them but that their intel was ignored. They uncovered information that could have prevented 9/11. But when the Able Danger team tried to tranfer that infromation to the FBI they were shutdown.
It seems the shutdown occurred because a similar datamining effort run by the Army at LIWA (Land Information Warfare) raised some serious concerns about Clinton and Gore's connections to Chinese front companies and the transfer of sensitive military technologies to China in exchange for campaign contributions to Gore and the DNC.
The Clintons did not want this coming out in the middle of Gore's 2000 Presidential run and all datamining efforts were shutdown, including this one which could have prevented 9/11.
It was then ignored by the 9/11 Commission as part of an ongoing coverup. When some whistelblowers within the DIA and DOD attempted to raise the issue, they suffered serious retaliation to contain the coverup.