"Traumatic brain injury of any severity is not strongly linked to aggressive behavior," he said.
U.S. officials have said the Army staff sergeant was trained as a sniper.
"To be a sniper, you've got to have the ultimate in impulse control. You have to move slowly, you have to inhibit, you have to wait ... to put the brakes on even in situations where the heart is pounding and it's a high-stress environment," O'Shanick said. To be chosen for such a role, he must have been able to control impulses better than the average person, O'Shanick said.
In prisons, there is a high percentage of inmates with traumatic brain injuries, he added. That usually doesn't make them more aggressive, but "when it goes bad, it can go bad significantly," he said.
AP writers Robert Burns in Washington and Mike Baker in Olympia, Wash., contributed to this report.
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP
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