"It is prudent to determine immediately whether there are internal weaknesses or procedural shortcomings in the department that could make us vulnerable in the future," said Gates, who spoke at a press conference at the Pentagon on Thursday afternoon.
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Hours earlier, the Senate Homeland Security Committee, led by Sen. Joseph Lieberman (D-Conn.) opened its own probe into the shooting.
"Ours looks forward and is preventive," said Lieberman. Lieberman said that while his committee's probe would respect Muslims, "We do no favor to all our fellow Americans who are Muslim by ignoring real evidence that a small number of their community have... become violent Islamists and extremists."
The new Pentagon probe will be led by former Army Secretary Togo West, who was Secretary of the Army and then Secretary of Veterans Affairs during the Clinton administration, and retired Admiral Vernon Clark, formerly Chief of Naval Operations. Secretary Gates said the review, which will last 45 days, will focus on gaps in the military's identification of personnel who may pose a threat.
The Homeland Security Committee's investigation, said Lieberman, will examine whether warning signs about Hasan's "mental stability and political extremism" were ignored by the Army, and how federal authorities responded after learning that Hasan had been exchanging e-mails with radical imam Anwar al Awlaki, described to ABC News by a former top CIA official as a recruiter for al Qaeda.
Gates was questioned about the Awlaki e-mails at the Pentagon press conference, and said he was disturbed by information that Hasan had contact with the American-born cleric, who is now based in Yemen. "But before I draw any conclusions about it," said Gates, "I want to find out all the facts." However, Gates said that the sharing of information between the Pentagon and the intelligence agencies is "dramatically different and dramatically better" than it was when he first left government in 1993.
Said Gates, "The most important thing for us now is to find out what actually happened, put all the facts together, and figure out a way where we can do everything possible so that nothing like this ever happens again."
"My message to all those in uniform, including Muslim in uniform, is how much we appreciate their service," said Admiral Mike Mullen, chairman of the joint chiefs of staff, who appeared at the Pentagon press conference with Gates. "The diversity of our force is one of its greatest strengths."