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Lieutenant-General Robert Cone, the base commander at Fort Hood, said that soldiers who witnessed the rampage heard him shout out the invocation as he opened fire.

General Cone told NBC's Today programme that Major Hasan was not known to be a threat or risk at the base. Colonel Braverman said the same.

However, Major Hasan had been transferred only a few months ago from the prestigious Walter Reed Army Medical Centre in Washington DC, possibly after he came to the attention of the FBI over an internet posting examining the motives of Islamist suicide bombers.

Of those killed, one was a civilian and 12 were soldiers. Major Hasan is in a stable condition on a ventilator.

The Army said the gunman opened fire at about 1.30pm local time at the Soldiers Readiness Processing Centre, a group of buildings where soldiers were having medical check-ups before leaving for overseas deployments.

The gunman had two weapons, one of them a semi-automatic, and neither was a military-issued weapon. Colonel Rossi said investigators were checking to see whether the weapons had been legally registered. There were only random searches of vehicles coming on to the base.

Colonel Braverman confirmed that the gunman had been due to leave for Afghanistan. One of Major Hasan's cousins said that he had been resisting any such deployment, which would have been "his worst nightmare".

An infantry sergeant at the Fort Hood base told The Times that Major Hasan had been well know for having an "agenda" as he counselled soldiers returning from action.

He added: "Put whatever politically correct whitewash on this you like, but there is no escaping this was a Muslim terrorist attack on US soil."

In an interview with The Washington Post, Major Hasan’s aunt, Noel Hasan of Falls Church, Virginia, said he had been harassed about being a Muslim in the years after the September 11 attacks and had wanted to leave the Army.

“Some people can take it and some people cannot,” she said. “He had listened to all of that and he wanted out of the military.”

A cousin, Nader Hasan, told The New York Times that after counselling soldiers returning from Iraq and Afghanistan with post-traumatic stress disorder, Major Hasan knew war first hand.

“He was mortified by the idea of having to deploy,” Nader Hasan said. “He had people telling him on a daily basis the horrors they saw over there.”

Federal law-enforcement agents ordered an evacuation of the apartment complex where Major Hasan lived in Killeen, Texas, last night before searching his home.

A local television station reported that the major had been giving away items of furniture and copies of the Koran yesterday morning.

Meanwhile, Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, appealed for supporters of the slain soldiers to respect a moment's silence later today.

On his Twitter feed, Admiral Mullen wrote: "Please join OUR military in a moment of silence & prayer 4 the fallen @ Fort Hood - TODAY @ 2:34pmET"

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