© The Associated Press/Allauddin KhanAn elderly Afghan man sits next to the covered body of a person who was allegedly killed by a U.S. service member, in a minibus in Panjwai, Kandahar province south of Kabul, Afghanistan, Sunday, March 11, 2012.
So the still-unnamed U.S. Army perpetrator of the Sunday slaughter in Afghanistan apparently suffered a traumatic brain injury two years ago. Could this have triggered what by all accounts was an irrational act? You better believe it, current and former Army mental-health professionals say.

While they stress that such an assessment in this case is premature, they agree that even mild traumatic brain injuries -- concussions, in other words -- can cause problems years later. "What you see often with TBI is a disinhibition," says an Army psychiatrist who recently served just outside Kandahar, near where the shootings took place. "TBI could be responsible if it leaves him predisposed to bipolarity or manic episodes."

The Army's one-time top psychiatrist concurs. "A mild TBI certainly can contribute to irritability and impulsivity, even two years later," says retired colonel Elspeth "Cam" Ritchie. "I don't think it would cause somebody to snap, but I think it could be a contributing factor, a straw that breaks the camel's back."

U.S. officials say a 38-year old staff sergeant, trained as a sniper, is now in custody. He arrived in Afghanistan in December following three earlier combat tours in Iraq. He's the lone suspect in the slaying of 17 civilians outside Kandahar in the middle of the night (the death toll originally had been 16, but the Afghan government has upped the count by one, and the U.S. government has agreed with the higher number).

The suspect suffered a TBI in Iraq in 2010 after a vehicle he was riding in rolled over in Iraq. But he passed a mental-health exam following the injury, and was declared fit to deploy at Joint Base Lewis-McCord outside Seattle, U.S. military officials say. But such mental-health scrutiny is imperfect and crude, especially if the soldier involved is eager to deploy, they add.

TBIs, along with post-traumatic stress disorder, are the two "signature wounds" of the wars in Afghanistan and Iraq. TBIs tend to be caused by the blasts from roadside improvised explosive devices, although they can be caused by other trauma, including vehicle accidents. Experts suggest perhaps one in five U.S. troops serving in post-9/11 conflicts have suffered a TBI injury.

The Army psychiatrist recently back from near where the shootings occurred, who declined to be named, said he knows of two cases near where he was deployed outside Kandahar where two soldiers walked off post. "They lost all sense of reality and they just marched down the road -- one even got to another FOB," he says. "It was kind of a-holy-shit-did-he really-do-that kind of moment. It was literally a death wish out there" (though neither died).

CNN has reported that the suspect has been moved to pre-trial confinement in Kandahar. The Army psychiatrist fears he is undergoing psychiatric testing there to determine his mental condition. "The evaluations that this guy is getting are most likely occurring in Kandahar, which is still getting rocket attacks," he says. "If someone is psychotic, and feeling threatened, can you really do an adequate test while they're still in the combat zone? It may be convenient for the legal system," he adds, "but it may not be appropriate."