The suspected shooter accused of killing 13 people at a US military base has regained consciousness and can talk, fueling hopes Monday he may soon reveal the motive behind the attack.

"He is talking. He is conversing with the medical staff," a spokeswoman for the Brooke Army Medical Center in San Antonio told AFP, referring to army psychiatrist Major Nidal Hasan.

Amid warnings that scores of US troops under stress from repeated tours of duty in Afghanistan and Iraq are falling through the cracks, commanders at Fort Hood, Texas, said they had ordered officers to keep a careful watch.

Officers must now keep an eye out for similar signs of disquiet "across our entire formation, not just in the medical community, but look hard to our right and left," said base commander Lieutenant General Robert Cone.

"That's the responsibility for everybody from the top to the bottom, to make sure we're taking care of our own."

A round-the-clock inquiry at Fort Hood has so far failed to uncover the motives for Thursday's shooting, which also left 42 wounded, according to a new toll released Monday.

Hasan, a devout Muslim, was shot and wounded by a female civilian police sergeant who halted his deadly gunfire, and had been in critical condition on a ventilator until Saturday.

So far investigators believe the 39-year-old acted alone, but new questions arose as to whether the shooting could have been a terror attack, amid reports he may have had links to an American-born imam who has backed Al-Qaeda.

The bloody spree has left army officials scrambling to understand how one of their own could turn on his fellow soldiers, prompting pledges of better monitoring in the future.

The shooting suspect "was a soldier," Cone told reporters, "and we have other soldiers that, you know, that might have some of the same stress and indicators that he has."

A hospital spokeswoman refused to say whether Hasan -- who is said to have been under tremendous hardship from counseling war-scarred soldiers -- had already been interviewed by army investigators.

President Barack Obama and First Lady Michelle Obama were due to travel to Fort Hood Tuesday for a memorial service, with some 5,000 people expected to attend.

Defense Secretary Robert Gates visited the base on Monday to meet the families of those killed and to visit some of the wounded, including police sergeant Kimberly Munley, hailed as a heroine for confronting the gunman.

Federal investigators are examining possible links between the army psychiatrist and Anwar al-Aulaqi, who was the spiritual leader of the Dar al-Hijrah mosque in Falls Church, Virginia, The Washington Post reported. Related article: More Fort Hoods waiting to happen

Hasan had attended the mosque in 2001, a year before Aulaqi left the United States and settled in Yemen.

The imam was said to have crossed paths with Al-Qaeda associates, including two September 11 hijackers, and is now believed to have become a supporter of the terror network, the paper said, citing a senior US official.

In a message posted on his website Monday, Aulaqi praised Hasan for his "heroic act," the SITE monitoring service said.

"In fact the only way a Muslim could Islamically justify serving as a soldier in the US Army is if his intention is to follow the footsteps of men like Nidal," said Aulaqi.

The mosque's current imam said some 3,000 people attended prayers every Friday and the congregation should not be judged by a handful of its members, adding that Hasan was not a regular worshipper there.

Senator Joseph Lieberman, who chairs the Senate Homeland Security Committee, has said he would launch a probe into whether the army missed any warning signs which could have prevented the attack.

"There are very, very strong warning signs here that Dr Hasan had become an Islamist extremist and, therefore, that this was a terrorist act," he told "Fox News Sunday."

His family has said Hasan had complained of harassment in the military and was deeply concerned about his orders to deploy to Afghanistan later this month.