Schools were closed, streets deserted, and checkpoints set up at entrances to Fort Hood over the weekend, as investigators tried to establish how a lone gunman was able to plan and carry out one of the most deadly killing sprees in US military history.

The death toll from Thursday's attack stands at 13, with another 30 injured during the incident that saw Major Nidal Malik Hasan open fire on a group of 300 unarmed soldiers waiting for check-ups at one of the base's medical facilities.

Doctors warn that the number of fatalities may still rise. Lt-Gen Bob Cone, the base commander, said the death toll could have been far higher had it not been for Sgt Kimberly Munley, a civilian police officer who arrived at the scene three minutes in, and disarmed Hasan, shooting him four times.

Munley was wounded by a bullet that passed through both her legs. Her actions were "amazing and aggressive", he said. "She was... one of our most impressive young policemen. She walked up and basically engaged him. I think, certainly, this could've been far worse."

Eyewitness reports suggest the attack unfolded shortly after 1.30pm, as the troops lined up for eye tests and inoculations at a group of buildings known as the Soldiers Readiness Processing Centre. The gunman was armed with two pistols, one of them automatic.

Cone spoke with several victims, one of whom told him: "I made the mistake of moving and I was shot again."

Officials are not ruling out the possibility that some casualties were victims of "friendly fire" from police officers. Cone admitted it was "counterintuitive" that a single shooter could hit so many people, though the massacre occurred in "close quarters" and involved "ricochet fire".

Since Hasan's religious convictions were seemingly a factor, the attack represents a fresh PR challenge for America's Muslim community. Barack Obama warned against "jumping to conclusions" about the massacre.