Defense Secretary Robert Gates named two former top military commanders to investigate "gaps and deficiencies" in programs aimed at finding troops who endanger colleagues. Gates said the 45-day probe would prevent "similar tragedies."

The hearing of the Senate Homeland Security and Governmental Affairs Committee marked the first step of what could be a congressional push to revise laws and policies and encourage authorities to share more information and to more aggressively investigate suspicious troops.

Sen. Susan Collins of Maine, the committee's top Republican, said military officials and FBI investigators failed to follow "red flags galore" about Maj. Nidal Hasan. The Army psychiatrist is charged with 13 counts of murder but has not been charged with terrorism.

"It appears we did have a failure to share critical information and a failure to ask critical questions," Collins said. "It reminds me very much of the siloed information that was available throughout the federal government in different agencies before 9/11."

An FBI-led terrorism task force looked into Hasan in December but did not tell the Pentagon that he had exchanged 10 to 20 e-mails with a radical Muslim cleric in the winter and spring. The task force decided the e-mails were related to Hasan's research. The FBI said it was barred from sharing information about Hasan.

Collins said the committee would "identify legal barriers that may have blocked the flow of information."

Committee chairman Joe Lieberman, I-Conn., defended his decision to hold the hearing despite President Obama's plea Saturday that lawmakers "resist the temptation to turn this tragic event into political theater."

"Their investigation looks backward and is punitive," Lieberman said of administration probes into intelligence about Hasan. "Ours looks backward and forward and is preventative."

The two-hour session elicited no information about Hasan. The witnesses were five terrorism experts, including Frances Townsend, a former homeland security adviser to President George W. Bush, who spoke about the rise of "homegrown" terrorists.

Sen. John McCain, R-Ariz., asked each witness whether the attack was terrorism. Three said yes, including retired Army general John Keane, who cited reports that Hasan was screaming "Allahu Akbar," or "God is great."

Brian Jenkins of the RAND Corp. resisted the conclusion, saying, "We've got him on an ordinary crime, and that's good enough."