This timeline is based partly on assertions by Rep. Curt Weldon and five military intelligence officers who say they participated in Able Danger. Pentagon officials have corroborated some of the assertions, declined to comment on others, and denied others:

Fall 1999: A top-secret military program called Able Danger is created. Twenty military intelligence officers and other team members begin using open-source computer data-mining to track al Qaeda and other terrorist groups.

January/February 2000: Able Danger identifies a Brooklyn cell of al Qaeda, including Mohammed Atta, later named as the leader of the Sept. 11, 2001 attacks, and three other hijackers. The program eventually identifies members of five al Qaeda cells around the world.

July/August 2000: DIA employees destroy all or some of the 2.4 terabytes _ 2,400 gigabytes _ of data collected by Able Danger.

October 2000: Lt. Col. Anthony Shaffer, an Able Danger member, meets with the Defense Intelligence Agency deputy director and offers him a computer disc with information about al Qaeda and Atta, but the DIA official declines to accept the disc.

January 2001: Able Danger leaders brief Gen. Hugh Shelton, chairman of the Joint Chiefs of Staff, on their findings.

Spring 2001: Able Danger program is abandoned.

Sept. 11, 2001: Al Qaeda operatives hijack four commercial jets, kill 2,986 people in New York, Washington and western Pennslyvania.

Sept. 25, 2001: Weldon and two other lawmakers meet at the White House with deputy national security adviser Stephen Hadley. Weldon later says he showed Hadley a chart developed more than a year earlier by Able Danger, diagramming 60 members of al Qaeda around the world and including Atta and the Brooklyn cell. Hadley later says he recalls seeing a chart at some point but doesn’t remember when or exactly what it showed.

Nov. 15, 2002: Over opposition from President Bush, Congress sets up a high-level commission to investigate the Sept. 11 attacks.

Oct. 21, 2003: Three staffers of the 9/11 commission, along with an unnamed “representative of the executive branch,” meet with Shaffer and two other Able Danger members at a U.S. air base in Afghanistan. Shaffer later claims that he told the 9/11 staffers that Able Danger had located, at least a year before the Sept. 11 attacks, four of the al Qaeda members who later carried out the hijackings and attacks, including ringleader Atta; 9/11 commission aides will dispute Shaffer’s version of the meeting, saying the Able Danger operatives didn’t cite Atta or other hijackers.

July 22, 2004: After a 20-month probe, the 9/11 commission releases a report on the 2001 attacks. The report casts blame broadly across the government for underestimating the threat from al Qaeda and makes sweeping recommendations for change, but doesn’t cite Able Danger or its findings, even to refute them.

Aug. 12, 2005: Thomas Kean and Lee Hamilton, the leaders of the 9/11 commission, release a statement on Able Danger, citing the 9/11 panel’s conclusion that Able Danger “did not turn out to be historically significant” and provided no strong documentation of its member’s claims.

August/September 2005: Weldon meets with military officers involved with Able Danger, who describe their findings.

Sept. 1, 2005: At a Defense Department briefing, Pentagon officials for the first time acknowledge the previous existence of Able Danger and say at least some Able Danger data had been destroyed.

Sept. 21, 2005: Senate Judiciary Committee holds hearing on Able Danger. Defense Department prohibits Able Danger officers from testifying.

September/October 2005: Senate Intelligence Committee hears closed-door testimony on Able Danger and promises to release a report on the program.