Investigators trying to understand why a US army psychiatrist launched into a shooting spree on a military base in Texas will be poring over an internet posting he is thought to have made comparing the sacrifice of Islamist suicide bombers and American military heroes.

Thirteen people were killed and more than 30 were injured when Major Nidal Malik Hasan opened fire at Fort Hood yesterday afternoon in a part of the base where soldiers are prepared for deployment to Afghanistan and Iraq.

The gunman himself was shot four times and was reported last night to be unconscious and on a ventilator in a nearby hospital. The female officer said to have shot him is also in hospital.

Major Hasan, 39, whose job involved counselling soldiers suffering from post-traumatic stress disorder after overseas tours, had himself fought a long and unsuccessful campaign against deployment to Iraq – which a cousin described as "his worst nightmare".

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The broken marriages, the mental afflictions, the occasional suicides and desertions: the Iraq War has had a huge impact on Fort Hood

Martin Fletcher

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As President Obama said in his first statement on the slaughter, the killings were all the more "horrifying" for having happened at an army base on American soil. But Major Hasan's background – he is a Muslim of Palestinian descent – prompted immediate speculation that the attack was a premeditated act of terror.

It was reported that Major Hasan had come to the attention of the FBI after a user named NidalHasan posted on the Scribd.com website in May, comparing the actions of an American soldier who threw himself on a grenade in Iraq with those of Islamist suicide bombers. No action was taken against him.

"He intentionally [sic] took his life (suicide) for a noble cause i.e. saving the lives of his soldier. To say that this soldier committed suicide is inappropriate. Its [sic] more appropriate to say he is a brave hero that sacrificed his life for a more noble cause," NidalHasan wrote.

"Scholars have paralled [sic] this to suicide bombers whose intention, by sacrificing their lives, is to help save Muslims by killing enemy soldiers. If one suicide bomber can kill 100 enemy soldiers because they were caught off guard that would be considered a strategic victory."

The posting was analysing rather than glorifying the actions of suicide bombers, but the distinction was lost in a slew of angry comments left after the news emerged of the slaughter in Texas.

"Not all Muslims are terrorists but all terrorists are Muslims," one commenter wrote. "Texas will fry your ass," wrote another.

Flags outside churches and cemeteries around Fort Hood were at half-mast as the occupants of America's largest military base tried to comprehend the killings.

Major Hasan had been armed with two non-military issue pistols, including a semi-automatic, as he entered the pre-deployment station together.

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