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(NECN) - At this time, there are few clues to what motivated Washington Navy Yard gunman Aaron Alexis.
Sources tell the Associated Press that investigators have found no manifesto or other writings suggesting a political or religious agenda, and the search for answers continues.
Jack Levin, the Brudnick Professor of Sociology and Criminology at Northeastern University, is the author of "Serial Killers and Sadistic Murderers: Up Close and Personal."
He says Alexis may have had a grudge against the Navy.
"I've seen that in almost every mass killing, whether it's a rampage shooting or a workplace murder or a family annihilation - there's always the idea of revenge," Levin says.
Watch the attached video for the complete interview.