By Daphne Benoit

Worshippers packed into churches Sunday seeking answers after a gunman opened fire in a US military base killing 13 and wounding 30, as army investigators appealed for help in their inquiry.

"This has been a truly horrific last three days and every person has been touched in some way by this tragedy," said garrison chaplain Colonel Frank Jackson at one of the services about Thursday's bloodshed.

"All those around us search for meaning, answers and for someone to blame and it's so frustrating," he said.

At the Comanche chapel, a largely African-American congregation sought refuge from the devastation at the Fort Hood base in Texas in gospel singing praising God's glory.

"Our focus today is on resiliency. The army is strong, we are focused on recovery," said chaplain Captain Jason Black.

Army investigators believe the suspected shooter, Major Nidal Hasan, 39, was acting alone when he opened fire in a troops processing center on Thursday.

"All evidence at this point indicates the suspect allegedly acted alone," said army investigation spokesman, Chris Grey, appearing to strike down any theories that Hasan was part of a radical Islamist sleeper-cell.

But as US President Barack Obama prepared to attend a memorial Tuesday, a US senator said he would launch a probe into whether the army missed any warning signs, saying the clues pointed to terrorism.

The chairman of the Senate's Homeland Security committee, Joe Lieberman, said the deadly shooting at the Texas base was an act of "Islamist extremism," although it was too early to spell out the motives behind 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 on Sunday.

"It's clear that he was, one, under personal stress and, two - if the reports that we're receiving of various statements he made, acts he took are valid - he had turned to Islamist extremism," he said.

"If that is true, the murder of these 13 people was a terrorist act."

The army's Criminal Investigation Command formally called for witnesses to come forward, asking for clothes or other personal effects that may contain gunshot residue to help put together a ballistic portrait of Thursday's fateful events at building 42003.

From their work, and the testimony of those who met Hasan, a picture has emerged of a deeply religious American of Palestinian descent, who opposed his country's prosecution of wars in Afghanistan and Iraq.

Hasan was to be deployed to Afghanistan later this month, army officials said.

An initial search of Hasan's computer revealed no direct exchanges with known extremists, but US Army and FBI officials had yet to rule out completely possible links to terrorist groups, US media reported.

Earlier this year, the Federal Bureau of Investigation learned of Internet postings by a man calling himself Nidal Hasan that expressed support for suicide bombings.

Witnesses reportedly heard Hasan, who investigators believe fired more than 100 rounds during the incident, shout "Allahu Akbar!" (God is greatest) before opening fire.

But US commanders, fearing a backlash against Muslim troops - prized for their cultural and linguistic knowledge - warned against jumping to any conclusions.

Army chief of staff George Casey warned deadly shootings at Fort Hood could prompt a backlash against the estimated 3,500 Muslims serving in the US military.

"I worry that the speculation could cause something that we don't want to see happen," he said.

Sixteen of the 30 people wounded by volleys from the semi-automatic and the handgun allegedly wielded by Hasan, remained in hospital on Sunday as doctors warned many would be permanently scarred by their injuries.

The suspect himself, who was shot by a female civilian police officer who was being hailed as a heroine, was taken off a ventilator, but was still in intensive care.