Major Nidal Malik Hasan, an army psychiatrist, faces 13 counts of murder and 32 counts of attempted premeditated murder for the rampage at Fort Hood army base on November 5.

Gates said he has forwarded to the army recommendations that some personnel responsible for supervising Hasan be held accountable following the attack.

"I would ask all commanders and leaders at every level to make an effort to look beyond their day-to-day tasks and be attuned to personnel who may be at risk or pose a danger," he said.

'Insider threat'

The Pentagon's two-month investigation into the failings behind the attack said that there were discrepancies between Hasan's performance and his personnel records.

"There is clearly more and more [self-radicalisation] going on, and how much of it we have in the military is something that we ought to really understand"

Admiral Mike Mullen,
chairman of the Joint Chiefs of Staff

"There is not a well-integrated means to gather, evaluate, and disseminate the wide range of indicators that could signal an insider threat," Retired Admiral Vernon Clark, who led the investigation, said.

Their investigation also found that his high-level security clearance had not been investigated properly.

Had policies been followed properly, investigators say, Hasan's clearance may have been revoked "and his continued service and pending deployment would have been subject to increased scrutiny".

Separately, the FBI said it would revise its own procedures to make sure that when it investigates a member of the military, it notifies the Pentagon.

In the Hasan case, a local joint task force run by the FBI with some military personnel examined Hasan but did not alert the defence department about the investigation.

After the shooting took place, it was reported that Hasan had been in contact with a Yemeni religious leader apparently sympathetic to al-Qaeda.

'Self-radicalisation'

Admiral Mike Mullen, the chairman of the joint chiefs of staff, said that "self-radicalisation" by individuals seeking out extremist views was of particular concern to the military.

"There is clearly more and more of that going on, and how much of it we have in the military is something that we ought to really understand," he said.

However, the question of how exactly the military could identify "self-radicalisation" among US forces remains an open question.

Authors of the Pentagon-ordered review ruled out sending spies into mosques, for example.

"Do we want commanders [eavesdropping] in the mosque? No. Do we want anybody there? No," Togo West, a former army secretary carrying out the review, said.

"What we want is commanders' awareness of what's happening in their units and what's happening with their people."