What those kinds of injuries can do to a person's decision-making is not totally certain. But scientists have linked brain trauma to some behavior, including violence.

A vehicle the accused staff sergeant was in rolled over in Iraq in 2010, a U.S. official told Reuters.

There is a link between brain injury and violence, according to researchers. And soldiers are at high risk for concussions and traumatic brain injury. Over 30,000 cases were diagnosed in 2011, according to the Department of Defense.

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"They're in a combat zone; it's a high-risk job," said Jordan Grafman, director of the Traumatic Brain Injury Research Laboratory at the Kessler Foundation in New Jersey.

David Hovda, director of the Brain Injury Research Center at UCLA and a member of the Department of Defense Health Board, was called to the Pentagon when the percentage of post-traumatic stress disorder and traumatic brain injury cases rose dramatically in the Army, according to the Washington Post ). 

Initially, the military psychiatrists and neurologists were upset, arguing that letting soldiers rest after a concussion would be bad medicine, Hovda said.

Since then, though, the military has begun to pay more attention to the mental injuries of war. "We just deployed three MRI scanners in Afghanistan this year," Hovda said.

"If [the accused soldier] had a mild TBI, he probably had more than one," Hovda said. "Most individuals don't tell the truth. When the brain's been hurt -- even mildly -- about 80 percent of individuals with mild TBI or a concussion recover in about two weeks."

Symptoms range from headaches to amnesia to sleep disturbances, anxiety, lack of judgment, and depression.

"In about 20 percent of individuals, those symptoms don't clear for months -- and we don't know why," Hovda said.

During that time period, the parts of the brain exposed to the injury become dysfunctional, he said. If the injury involves the frontal lobes of the brain, two things happen:

First, a loss of executive function. "That's a fancy term for trying to plan things or being able to make decisions," Hovda said.

Second, people become uninhibited.

"We have learned how to behave ourselves in public, so we may not shout out expletives in a radio interview or we may not accost someone because we don't like them," Hovda said. "When you have an injury to the frontal lobe, that's gone."

A study of the aggression pattern of Vietnam veterans shows how the effects of brain injuries often play out. When Grafman and colleagues studied the link between brain damage and aggression in Vietnam war veterans, they turned to the soldiers' wives.

The reports of increased yelling, throwing things, threats and physical violence were higher in veterans who had brain damage to the frontal lobes (a common area to be injured with a TBI) -- although physical violence was the least common form of aggression.

"The frontal lobe is a place where we store memories that have to do with appropriate social behaviors in different situations," Grafman said. "If the tissue that stores those memories is damaged, instead of being able to easily retrieve those social rules and memories to guide your controlling behaviors, you're more likely to be dependent upon intact, but more primitive brain structures, which makes an impulsive reaction more likely to provocation."

Also, Grafman noted, the study analyzed behavior in normal daily life.

"I don't have any data, and I don't know if anyone does, on aggression in people with brain injuries who have been returned to the combat zone," he said. "Would that increase the likelihood (of aggression) because of the world you're in?"

Some have suggested a link between post traumatic stress disorder and concussions and mild TBI. Some of the symptoms overlap, but it's probably not a causal relationship, Grafman said.

"The same situation where you might have received the injury in combat could also increase the likelihood of PTSD," Grafman said. "If people around you are being shot or you are shooting somebody, I can't imagine something more traumatic to experience. But, you can have brain injury and not have PTSD."

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"We made a DVD for soldiers on what a concussion is, and how it makes it more dangerous to your buddies and the civilian population," Hovda said. "You might be making decision on airstrikes that require seconds to make. And if your brain is compromised, you're not going to do very well and could really hurt someone."

There is some good news, however. Most people who experience concussions and mild TBI do not turn violent. And most brain injuries can be managed through cognitive behavior therapy and/or medication, and help from family, Grafman said.