Soldiers move into the Soldier Readiness Centre on Fort Hood. Photo / AP

Soldiers move into the Soldier Readiness Centre on Fort Hood. Photo / AP

Army investigators working to reconstruct in second-by-second detail the one-man rampage inside a soldier processing facility at Fort Hood, Texas, on Thursday, are asking how the assailant had time to fire at least 100 rounds from a single handgun. The incident left 13 dead and more than 30 wounded.

Some survivors of the carnage, made even more deadly by ricochet bullets bouncing from walls, furniture and floor tiling, have told investigators that the suspect, Major Nidal Malik Hasan, an army psychiatrist who faced imminent deployment to Afghanistan, was almost methodical in his slaughter. In some instances, he fired a second time at soldiers who were wounded but not yet dead.

Officials said Hasan bore two non-military handguns on entering the building. But after climbing first on to a desk and shouting "Allahu Akbar!", or "God is great!", he used only one, a Belgian semi-automatic pistol with an extended magazine that holds 20 bullets.

This implies that Hasan reloaded as many as five times before he was brought down by two civilian police officers.

Hasan, 39, was transferred to a San Antonio hospital last night. He remained unconscious and according to sources has been paralysed by his wounds. His condition was serious but stable.

Officials said Hasan was hit at least four times by the two police officers.

One survivor, Keara Bono, recalled yesterday that at first she thought the incident was a drill. She was among scores of personnel inside the building which is used to screen soldiers returning from duty abroad and to prepare others before their deployments, notably with health checks and vaccinations.

"Then I looked to my left and right and I saw people bleeding," Ms Bono, who suffered a graze to the head and a light back wound, told ABC News.

That's when she knew the attack was not pretend.

"I started crawling. I turned back and looked at him. He was about two body lengths away from me, shooting people on the ground. So I was just waiting to get shot in the back."

Another who was hit, Grant Moxon, came out alive only because he played dead, his father said in interviews.

"He said he looked up, and this guy is standing there looking at him right in the face. All of a sudden, he pulled up a gun and started shooting."

Dedicating his weekend radio address to the tragedy, President Barack Obama called it "all the more heartbreaking and all the more despicable because of the place where it occurred and the patriots who were its victims".