Twelve soldiers and a civilian were killed and 42 people wounded in the assault at the US military base, and commander Lieutenant Robert Cone Monday urged officers to be on the lookout for any troops showing signs of stress.
The Federal Bureau of Investigation has said Hasan, who Monday regained consciousness after the shooting, came to its attention in 2008 after he communicated with the target of a counter-terror investigation.
But US reports said hospital officials had considered sacking the army psychiatrist due to his lackluster performance and that he had unsettled colleagues with a presentation about the divided loyalties of Muslim soldiers.
His fellow psychiatrists told National Public Radio they were troubled by his work performance and that officials discussed sacking him from his job at Walter Reed Army Medical Center, outside Washington.
One hospital official said Hasan -- a devout Muslim with misgivings about the US wars in Iraq and Afghanistan -- had tried to convert a patient to Islam, telling the patient his religion would save him.
Hasan also warned a roomful of senior army physicians a year and a half ago that to avoid "adverse events" the military should allow Muslim soldiers facing missions in Islamic countries to be released from duty as conscientious objectors, the Washington Post reported.
"It's getting harder and harder for Muslims in the service to morally justify being in a military that seems constantly engaged against fellow Muslims," he was quoted as saying.
Hasan has emerged from a coma after being wounded by police gunfire, but has declined to speak to investigators and asked for his lawyer. Officials said he would likely be tried in a military court where, if convicted, he could face the death penalty.
The FBI said "the investigation to date indicates that the alleged gunman acted alone and was not part of a broader terrorist plot."
But US media reports said he had links to a radical imam, identified as Anwar al-Awlaqi, a former spiritual leader of a mosque in the Washington suburb of Falls Church, Virginia attended by Hasan.
Awlaqi has reportedly met Al-Qaeda associates, including two September 11 hijackers, and is believed to be a supporter of the group.
Federal investigators concluded Hasan's communications with the suspect were consistent with his research as an army psychiatrist and that he was not part of a terror network.
But some US lawmakers said the shooting increasingly resembled a "terrorist act," even if the suspect did not act on direct orders.
"I think it's very clear he has influence from the outside," Peter Hoekstra, the senior Republican on the House Intelligence Committee, told CNN television.
Referring to Awlaqi's web writings, Hoekstra said "it was part of his strategy to try to identify people in the US military and incite them to attack their fellow soldiers."
Hoekstra has called on the FBI and intelligence agencies to preserve all relevant documents in the case, and complained the agencies were failing to brief lawmakers in a timely manner.
But intelligence officials denied the lawmaker's allegation.
Director of national intelligence Dennis Blair "has been in communication with the leadership of the congressional intelligence oversight committees and will ensure there is a full accounting of the facts," said his spokeswoman, Wendy Morigi.
FBI director Robert Mueller, after meeting with President Barack Obama on Monday, ordered a full review of the shooting to determine whether "with the benefit of hindsight, any policies or practices should change based on what we learn," the FBI said.
Senator Joseph Lieberman, who chairs the Senate Homeland Security Committee, has vowed an investigation into whether authorities missed any alarm signals that could have prevented the assault.
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Image: President Barack Obama and First Lady Michelle Obama by the Fallen Hero Memorial at Fort Hood (AFP/Paul J. Richards).