Hasan attended the Dar al-Hijrah mosque in Falls Church, Va., in 2001, when its spiritual leader was Anwar al-Aulaqi, a figure who crossed paths with al-Qaeda associates, including two Sept. 11, 2001, hijackers, a senior U.S. official said.

Since leaving in 2002 and settling in Yemen, lectures by Aulaqi promoting the strategies of an al-Qaeda military leader have shown up in computer files of suspects in terrorism cases in the United States, officials said. It is not clear whether Hasan knew the preacher well or only later through his Internet lectures.

A federal law enforcement official said yesterday that investigators' operating theory remains that Hasan acted alone and without provocation or exhortation from an overseas person. But new leads are being pursued based on information gleaned in a review of Hasan's computer and multiple e-mail accounts. Those include visits to Web sites espousing radical Islamist ideas, the official said.

A challenge for investigators is sorting out a potential thicket of psychological, ideological, or religious motivations behind Hasan's alleged actions. Hasan's possible contact with extremists such as Aulaqi would complicate matters, suggesting that U.S. authorities may have missed chances to prevent the cleric from instigating this incident and others.

But if Hasan acted in the throes of an emotional breakdown, his questionable ties could be misinterpreted in ways that damage U.S. outreach to the Muslim world.

"There's a massive effort here to look at the Web sites he visited," the law enforcement official said, speaking on the condition of anonymity because of the ongoing probe.

Shaker Elsayed, the senior imam at Dar al-Hijrah, a long-established mosque, said Hasan had prayed there since 2008 and sought his help to find a wife, but he could not verify whether Hasan met Aulaqi. Supporters of the mosque note that Aulaqi spent only a year there, publicly condemned the Sept. 11 attacks, and was not known to give radical speeches at that time.

Yesterday, Army Chief of Staff George Casey cautioned troops against jumping to conclusions about what might have motivated Hasan, an Army psychiatrist, to allegedly shoot and kill 13 people and wound 38 Thursday at the nation's largest Army post. "I'm concerned that this increased speculation could cause a backlash against some of our Muslim soldiers. And I've asked our Army leaders to be on the lookout for that," Casey told CNN's State of the Union.

Sen. Joe Lieberman, (I., Conn.), chairman of the Senate Homeland Security and Governmental Affairs Committee, said he planned a congressional investigation to determine whether the shootings constituted a terrorist attack and whether the Army missed warning signs about Hasan's ideological views. The House Armed Services and Intelligence Committees are also likely to investigate.

Hasan, 39, born and raised in Virginia, attended Dar al-Hijrah at the time of his mother's May 2001 death. In recent years, he worshiped at the Muslim Community Center in Silver Spring, Md., which is regarded as moderate.

Aulaqi, whom one official called "a radicalizer of the first order," has been identified as a spiritual adviser of 9/11 hijackers Khalid Almihdhar and Nawaf Alhazmi; the 9/11 Commission Report noted they met with Aulaqi at a mosque in San Diego in 2000 and after he moved to Dar al-Hijrah in 2001.

The FBI investigated Aulaqi nearly a decade ago, after he briefly served as vice president of the U.S. branch of a Yemeni charity that federal prosecutors later described as a front organization used to support al-Qaeda and Osama bin Laden.

The FBI also learned that Aulaqi was visited in early 2000 by a close associate of Omar Abdel Rahman, the man known as "the blind sheikh" who was convicted in connection with the 1993 World Trade Center bombing.