Sergeant Kimberly Munley was the heroine America desperately sought in the hours after the massacre at the Fort Hood army base in Texas.

But a week after Major Nidal Hasan allegedly shot and killed 13 people and wounded dozens more, it has emerged that a previously unheralded police sergeant may in fact have fired the bullets that ended the attack, not Munley.

A witness to the shooting spree told the New York Times today that Hasan was taken down by Senior Sergeant Mark Todd, a veteran police officer.

The revelations came as the US military announced today that it had charged Hasan with 13 counts of premeditated murder amid growing questions about the failure of US authorities to arrest Hasan before the massacre.

The White House said today that President Barack Obama had ordered an internal review of intelligence gathered on Hasan prior to the attack. Obama said the inquiry by John Brennan, an adviser on homeland security and counterterrorism, would not interfere with the criminal investigation.

In TV appearances, Munley and Todd have thus far failed to clear up exactly who shot him.

Munley insisted that she fired at Hasan, but did not say whether she had hit him. Todd talked of firing the shots that brought down Hasan - the first time he had ever fired at a human being in his 25-year career - but could not say whether Hasan had already been shot.

"I seen him [sic], I drew his attention toward me, and then he opened fire and then I neutralised him and secured him," Todd told CBS's Early Show this morning.

Pressed whether Hasan was still armed and threatening when he opened fire, Todd said: "We were engaged in a gunfight, and then I neutralised him, or we neutralised him."

Hasan, 39, is recovering in the hospital from police gunshot wounds. Ballistics tests and further investigation may ultimately determine which of the two police sergeants shot him.

The tale of Munley's apparent heroism recalls the case of Jessica Lynch, an army private first class who the US military said was wounded while fighting off Iraqi troops during the US invasion, reportedly firing her rifle heroically before being captured. It later emerged she was injured in a vehicle accident during an ambush and was well cared for by the Iraqis.

Munley, a petite, highly-trained civilian police officer and firearms expert, was initially said to have encountered Hasan during the attack, rushed at him and shot him down, taking at least three bullets from his pistol in the process.

Quoting an anonymous eyewitness, however, the New York Times called into question that account, reporting that Hasan remained on his feet, apparently unharmed and reloading his pistol, after shooting Munley. The witness said it was unclear whether Munley fired off a shot before Todd arrived seconds later, shooting Hasan until he fell to the ground then kicking his gun away and handcuffing him.

"It's muscle memory," Todd told CBS. "In a situation like this you don't have to think about it, you just react. By the grace of god I'm standing here with no injuries, with not a scratch."

Contrary to previous reports, Hasan's military personnel record indicates that he did not apply for an early discharge from the army, as a conscientious objector or for any other reason, according to military officials. Hasan, who was born in the US to Palestinian parents, was widely understood to have sought and been denied an early separation from the army, a factor that could have helped explain why he allegedly snapped and went on the shooting rampage.

Family members have said that Hasan sought desperately to leave the army, especially ahead of an expected deployment to Afghanistan. A cousin in the West Bank has said that as recently as a week before the incident the army rejected Hasan's effort to gain an early separation. The army, short on psychiatrists and desperate to retain Arab-Americans among its ranks, was highly unlikely to accede to such a request from Hasan, having paid for his medical training.

But the Pentagon has found no evidence Hasan ever formally sought to leave the military. Another indication Hasan had not put in for a discharge: He was promoted to the rank of major in May, after an army board endorsed his performance as an officer, the Washington Post reported.

Meanwhile, officials involved in Hasan's medical training have described him as belligerent, defensive and argumentative when discussing his apparently deeply held Muslim faith.

Officials also described him as a mediocre student and a lazy worker, and said the combination led to a decision to send him to Fort Hood, where other medical personnel could pick up the slack if he continued to perform poorly and superior officers could document behaviour problems, the Associated Press reported.