Fort Hood, Texas  - As investigators tried to determine the motive behind the shooting rampage at Fort Hood, the largest US military installation in the world, mourners struggling to cope with their loss held a candlelight vigil to commemorate the dead.

The alleged gunman in the attack that killed 13 people, army psychiatrist Major Nidal Malik Hasan, remained in critical condition late Friday but was expected to survive. The 39-year-old devout Muslim was transferred to the Brooke Army Medical Centre in San Antonio, Texas, where he remained on a ventilator and was unable to communicate, an army spokesman said.

Of the 30 people wounded in Thursday's attack at Fort Hood, 23 were still in hospital, Colonel John Rossi said Friday night.

"We have secured the installation and the focus is now on healing and transition after this tragedy," Rossi said.

In the soul searching that has followed the attack on the ravages of war and the stress that soldiers live with, authorities have focused on destigmatizing the need for psychological help.

"We want to remove the stigma from soldiers and their leaders who come forward and say, 'I need some help,'" said Rossi, adding that a 24-hour grieving centre had been set up at Fort Hood with clergy and counsellors.

Hasan reportedly shouted, "Allahu Akbar," or "God is great," before unleashing two handguns at the Texas base's centre that prepares soldiers for deployment overseas. Rossi said he fired more than 100 rounds.

President Barack Obama cautioned against any quick judgements about the shooter's motive: "We don't know all of the answers yet, and I would caution against jumping to conclusions until we have all of the facts."

Obama ordered the US flags of all federal government buildings to be flown at half-staff to mourn the victims - 12 soldiers and one civilian police officer.

"This is a modest tribute to those who lost their lives even as many were preparing to risk their lives for their country," Obama said.

Meanwhile, details were emerging about Hasan as family and friends expressed shock that he could have been behind the crime but also that he had grown disgruntled by his pending deployment to Afghanistan.

Colonel Catherine Abbott, an army spokeswoman, confirmed Friday that Hasan was being deployed to Afghanistan but could not provide a timeframe for his departure. He was to join a reserve unit for treating combat stress.

US media reported that Hasan had sought to be discharged from the army, and relatives said he was horrified about the possibility of going to Iraq or Afghanistan and had grown tired of being harassed by other soldiers because of his faith. Hasan reportedly opposed the wars in Iraq and Afghanistan and was not shy about conveying his position to co-workers and friends.

As an army psychiatrist, Hasan had treated soldiers returning from both conflicts for post-traumatic stress disorder and other ailments. He was facing his first deployment to a war zone since he joined the army in 1995.

Born in Virginia, Hasan grew up in the Washington area and frequented the Muslim Community Centre in Silver Spring, Maryland, which borders the nation's capital.

"I've known him since 2007," the centre's imam, Mohammed Abdullahi, told the German Press Agency dpa. "He was quiet, normal. There was nothing crazy about him like everyone is making it out to be. He didn't look crazy."

At Friday prayers, he offered condolences to the families of the victims. "This is a very open-minded community, and all of us are saddened by what happened, but a particular faith cannot be held responsible," he said, adding that a multifaith conference would be held at the centre Monday.

The shootings shocked a military base accustomed to grieving for soldiers killed abroad. The rampage also brought the war closer to home and highlighted the growing stress on the men and women who have been repeatedly deployed to Iraq and Afghanistan for extended combat duty during the past six to nine years.

"This was a kick in the gut," General George W Casey Jr, army chief of staff said, adding that the army has been working to improve mental health care and to develop methods for detecting behavioural problems.

The mental health of soldiers has become an increasingly high priority for the US Army, which has been coping with a high number of suicides from soldiers who returned from battle. (doa)