Fort Hood, Texas  - Chaplains offered words of comfort at Sunday services and tended to the spiritual needs of a flock shocked by a mass shooting at the US Army's Fort Hood.

Investigators continued to seek answers in the shooting of soldiers preparing to deploy overseas, in which 13 people were killed and 30 wounded. Authorities issued a call Sunday for soldiers who were present at the attack to come forward with evidence such as bullet casings or holes in belongings that could provide clues to help reconstruct the crime scene.

Chaplain Frank Jackson led his congregation in prayer at one of Fort Hood's nearly dozen chapels, where stained-glass windows depicted images of soldiers and doves of peace. With the pews of his small chapel packed with about 150 people, he acknowledged that there may never be satisfactory answers about the horror of a lone gunman opening fire on soldiers.

"I don't think there's an answer why," Jackson said, standing under gray, drizzling skies on the sidewalk in front of his small, yellow-brick chapel before beginning the service.

He lifted up the victims and their families but also offered prayers for the gunman, Army psychiatrist Major Nidal Malik Hasan, 39, and for his family as they struggle to "explain the unexplainable."

Wearing a dark blue, civilian business suit, Jackson prayed that the Army community would "not be overwhelmed by grief and sadness."

His sermon focussed on gathering together to get through tragedy.

Seventeen victims remained hospitalized late Saturday, 10 of them in intensive care, Colonel John Rossi told the reporters who have converged on Fort Hood.

Senior US Senator Joe Lieberman called Sunday for a congressional investigation to establish whether the mass shooting was motivated by terrorism.

There had been "strong warning signs" that Hasan was an "Islamist extremist," Lieberman told Fox News television.

"If that is true, the murder of these 13 people was a terrorist act and, in fact, it was the most destructive terrorist act to be committed on American soil since 9/11."

Hasan, a devout Muslim, could have been radicalized, and the probe should determine whether the Army missed any clues that he held extreme views, said Lieberman.

At Fort Hood, most of the community did not have time to dwell on the motives of the attack.

Instead, the 20th Engineering Battallion, which had four of its soldiers killed and 11 others wounded in the attack, spent Sunday delivering care packages of food as well as homemade meals to victims' families.

"It's very surreal to wake up and have something happening not in Iraq or Afghanistan but on the home front," said Lieutenant Colonel Pete Andrysiak.

But as his soldiers prepare to deploy in January to Afghanistan, this serves as a powerful reminder that the community will take care of the families if something were to happen to them.

"This is what the Army's all about," said his wife, Kasey.

She said the care team with which she volunteers was accustomed to helping families of soldiers in Iraq and Afghanistan, "but never do you think you're going to do a terror attack." She describe Fort Hood as horrified and stunned, but not wasting time with anger toward the gunman.

Secret Service agents were seen Sunday on the base, preparing security arrangements for President Barack Obama's visit to attend a memorial service on Tuesday.

Army officials on Saturday released the names of the 13 people killed in the attack before holding a brief moment of silence.

Hasan remained in the intensive care unit of the Brook Army Medical Centre in San Antonio, Texas but has been removed from a ventilator.

Chris Grey, a spokesman for the Army's Criminal Investigation Unit, said investigators have found no evidence to contradict the appearance that Hasan acted alone when he allegedly opened fire with two hand guns. Grey stressed that no motive had yet been established for the attack.

Hasan was to have deployed in late November to Afghanistan but had voiced opposition to the war. He reportedly shouted, "Allahu Akbar," or "God is great," before emptying two handguns - firing more than 100 rounds - at the Texas base's centre that prepares soldiers for deployment overseas.

Investigators have conducted more than 170 interviews and were at work reconstructing the scene. There was no evidence that any of those who died had been killed by those responding to the attack.

The shootings shocked a military base accustomed to grieving for soldiers killed in combat. The rampage brought the wars 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 last six to nine years.

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 by soldiers who returned from battle. (dpa)