Defence Secretary Robert M Gates on Thursday announced a Pentagon review of the shootings at Ford Hood, Texas, to help ensure that "nothing like this ever happens again."

A former Army secretary, Togo West, and Admiral Vern Clark, the former chief of naval operations, are to lead the 45-day review.

Hours earlier, a Senate committee opened the first public hearings into the shootings, with several legislators asserting that the November 5 attack, in which an Army psychiatrist is accused of killing 13 people and wounding 43 others, was a terrorist attack by a home-grown extremist who may have slipped past law enforcement and military authorities.

Gates said the goal of the review was threefold: to find any deficiencies within the Defence Department for identifying service members who might be a threat; to assess the military's mental health and counselling programs, among others; and to examine how the department responds to "mass casualty" events at its facilities.

"We do not enter this process with any preconceived notions," Gates said at a news conference. "However, 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."

He said that the 45-day review was the beginning of extended military reviews of the shootings.

The Army's chief of staff, General George W Casey Jr, is organising a military panel to examine whether warning signs were overlooked by the Army authorities at Fort Hood or the Walter Reed Army Medical Centre, where the psychiatrist, Major Nidal Malik Hasan, was stationed until July.

The 45-day review will shape a longer follow-up Pentagon examination of any institutional shortcomings, Gates said. The longer Pentagon review would take four to six months, he said.

Gates, who has so far said little in public about the shootings, described his initial reaction as one of "horror." He said "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."

Asked if he considered the shootings at Fort Hood a terrorist attack by Hasan, a Muslim whose opposition to the wars in Iraq and Afghanistan appeared to become much more radical in the last several months, Gates said, "I'm just not going to go there," and "I am going to wait until the facts are in."

Senator Joseph I Lieberman, a Connecticut independent who is chairman of the Homeland Security and Governmental Affairs Committee, said the congressional hearings would try to determine whether federal agencies and employees "missed signals or failed to connect the dots in a way that enabled Hasan to carry out his deadly plan."

Lieberman's hearing made only limited headway because the Obama administration has refused his requests for witnesses from the FBI and the Defence Department.

Lieberman said he had spoken with Attorney General Eric H Holder Jr and Gates, who told him they would cooperate with his inquiry, but did not want to compromise the criminal investigation.

As a result, Lieberman proceeded with several nongovernment experts and former officials, including Frances Fragos Townsend, formerly the homeland security adviser to President George W Bush.

Townsend expressed concern that "political correctness" and fear of intruding on Hasan's free speech rights might have interfered with the sharing of information earlier this year, when an FBI-led counterterrorism team examined his e-mail exchanges with Anwar al-Awlaki, a well-known radical Islamist cleric, but found nothing amiss.

Gates said that he found Hasan's contact with Awlaki "disturbing," but "before I draw any conclusion about it I want to find out all the facts."