The military psychiatrist responsible for a shooting spree that left 13 dead remains in a coma, frustrating attempts to work out what made him kill.

Major Nidal Malik Hasan was on Friday night said to be in a critical but stable condition as many of his victims continued to fight for their lives in hospital.

Meanwhile investigators resumed their search for a motive to the shooting at Fort Hood, Texas. Witnesses are said to have heard the suspect cry "Allahu Akbar" - Arabic for "God is great" - before opening fire at the sprawling military complex in Texas.

Alleged links to extremist websites were being examined as authorities attempted to work out if Hasan's rampage was driven by mental breakdown or ideological rage.

US Army Sgt Maj Leroy Walker Jr sheds a tear during a candlelight vigil at Fort Hood (AP) Full Image

Hasan, a devout American-born Muslim, was shot several times in the aftermath of the rampage by a civilian police officer who was on Friday hailed as a hero.

Lieutenant General Bob Cone said Sgt Kimberly Munley and her partner responded to reported gunfire within three minutes. She then disabled the suspect despite being shot herself in the process.

"It was an amazing and an aggressive performance by this police officer," the military spokesman said.

Shortly before her arrival, Hasan - armed with two non-military issue pistols, including a semi-automatic - entered a section used to prepare soldiers for deployment overseas and started firing.

Within minutes, 12 soldiers and a civilian at the base were dead or dying in the worst mass shooting yet at a military base in the US.

At least 30 others were injured in an attack described by President Barack Obama as a "horrific outburst of violence".