The man who turned guns on his fellow soldiers is an army psychiatrist trained to help his comrades come to terms with precisely the same scenes of terror that he himself brought to Fort Hood yesterday.

Major Nidal Malik Hasan, 39, is a mental health professional assigned to a base that is home to the US Army's "warrior reset programme", set up to help soldiers returning from war zones cope with post-traumatic stress disorder (PTSD).

He attracted the attention of police six months ago for internet postings analysing the motives of suicide bombers but was not formally investigated. Major Hasan had also complained to relatives of harassment by fellow-officers mocking him for "being Middle Eastern", according to a cousin who came forward last night.

Born and educated in Virginia, Major Hasan is a US citizen whose parents came from a small Palestinian town near Jerusalem.

What stress or anger motivated him to kill 13 people before being shot himself is a mystery as he lies, unconscious, on a hospital ventilator. One senior general there said that Major Hassan had been troubled about having to deploy to Iraq, where he was headed on November 28.

"He had spoken in the past about being upset about going to the war," Senator Kay Bailey Hutchison of Texas said after being briefed by the general. "He was about to deploy. He was upset."

Faizul Khan, a former imam at a mosque Major Hasan attended in Silver Spring, Maryland, said he is a lifelong Muslim and attended prayers regularly, often in his army uniform.

Officials said last night that Major Hasan was transferred to Fort Hood in July after working for six years at the Walter Reed Medical Centre in Washington. The vast military hospital is the country's leading rehabilitation centre for wounded soldiers returning from foreign wars.

Like Fort Hood, it has taken a lead in improving the US military's care of PTSD sufferers, but Major Hasan's performance evaluation there was poor and he may have been the subject of disciplinary action, officials said.

While on the East Coast he also worked as a "fellow in disaster psychiatry" at the Centre for the Study of Traumatic Stress at the Uniformed Services University in Bethesda, a Washington suburb.

His immersion in the field of PTSD, linked to a mounting incidence of violence and suicide among returning troops, is not the only cruel irony in the story he brought to such a bloody end.

Major Hasan is a graduate of Virginia Tech University, the scene of the worst massacre by a single gunman in recent US history when 32 people were killed there in 2007. The Fort Hood gunman left the university in 1997 with a degree in biochemistry. He is single, with no children.

He had complained of abuse by his comrades, his cousin said last night.

"He was dealing with some harassment from his military colleagues," Nader Hasan told last night. "I don't think he's ever been disenchanted with the military. It was the harassment." He had received "flak" for being Middle Eastern, but was also apprehensive about going to Iraq.

Since 9/11, being deployed was his "first fear," the cousin said.

The real story behind yesterday's tragedy may never be known, but the Pentagon was already redoubling its efforts last night to ensure that it is not repeated. As extra counseling staff were flown the the base, it emerged that the Army paid most of Major Hasan's college fees, committing him to at least five years' military service on graduating.

He once hired a lawyer to help him wriggle out of his contractual obligations to the army. He did not win.

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