An official investigation suggests Maj. Nidal Hasan was promoted despite documented complaints about his strident religious views and unprofessional behaviour. (Associated Press)

An investigation into the Fort Hood shooting spree found that doctors overseeing Maj. Nidal Malik Hasan's medical training repeatedly expressed concerns over his zealous religious views and unprofessional behaviour, but continued to give him positive performance reviews, according to information received by The Associated Press.

Hasan, 39, was charged with murdering 13 people on Nov. 5, 2009, at Fort Hood in Texas. It was the worst shooting rampage on a U.S. military base.

He had been working at Fort Hood as a military psychiatrist specializing in preventive and disaster psychiatry.

The day of the attack, according to witnesses, Hasan walked into the base's processing centre, jumped on a table with two handguns, shouted "Allahu Akbar!" — Arabic for "God is great!" — and opened fire.

Following the attack, U.S. Defence Secretary Robert Gates appointed two former army officials to conduct an investigation of the shooting. The information, collected by the Pentagon and obtained by The Associated Press, has not been released publicly and is supposed to be presented to Gates later this month.

The review revealed that Hasan's supervisors during his psychiatry internship and residency at Walter Reed Army Medical Center in Washington, D.C., from 2003 to 2007, voiced concerns about him in memos, meeting notes and counselling sessions.

Records from his medical school days at a Maryland university also indicated Hasan received average and failing grades and at one point had been placed on academic probation.

In the latter stages of Hasan's lengthy medical education in the Washington, D.C., area, he gave a class presentation questioning whether the U.S.-led war on terror was actually a war on Islam. Fellow students said he suggested that Shariah, or Islamic law, trumped the U.S. constitution and he attempted to justify suicide bombings.

In another incident already made public, a patient of Hasan's with suicidal and homicidal tendencies walked out of the hospital without permission.

Hasan was also cited for inappropriately engaging patients in discussions about religious issues.

Early in 2007, Maj. Scott Moran became director of psychiatry residency at Walter Reed and took a firm stance with Hasan. Moran disciplined him for not being reachable when he was supposed to be on-call, developed a plan to improve his performance, and informed him his research project about the internal conflicts of Muslim soldiers was inappropriate.

Nonetheless, Hasan presented the project, he titled: Koranic World View as It Relates to Muslims in the U.S. Military, and it was found to meet residency program requirements, according to the review.

Lower standards in medical corps: former official

Despite concerns about Hasan's behaviour, supervisors gave him positive reviews in his officer evaluation reports. Known as OERs, the reports are used to determine promotions and assignments. Hasan was promoted to captain in 2003, graduated from Walter Reed in 2007, and was promoted to major in 2009.

Upon graduation, Hasan received a favourable reference letter from Moran, despite the director's earlier concerns.

Reached by telephone, Moran declined to comment.

Hasan completed a two-year fellowship at the Uniformed Services University of the Health Sciences in Bethesda, Md., before arriving at Fort Hood in July 2009.

Some have speculated that Hasan was able to rise through the ranks, despite his superiors' hesitations, because the U.S. army is contending with shortages of both psychiatrists and majors in the medical corps.

"The army is caught in a perfect storm," Larry Korb, the Pentagon's top manpower official during the Reagan administration, told the Houston Chronicle soon after the Fort Hood shooting. "It's had to lower standards to keep people coming in — and it's made changes to keep people from getting out."

Defence Secretary Robert Gates is scheduled to receive the complete review by Jan. 15.

Army Lt. Col. Jonathan Withington, a Pentagon spokesperson, declined to comment on the review because it's not complete. "We will not know the specific content of the report until it is submitted to the secretary of defence," he said.

Hasan has been charged with 13 counts of murder and 32 counts of attempted murder. He remains at a San Antonio military hospital, undergoing rehabilitation for paralysis stemming from gunshot wounds suffered when police officers fired back during the massacre.

Authorities have not said whether they plan to seek the death penalty.

With files from The Associated Press