The man who investigators say fatally shot four marines at a Navy-Marine training center and opened fire at a military recruiting center in Tennessee had a bachelor's in engineering and was an outgoing, popular student in high school who belonged to the wrestling team.

But beyond that, not much is known about Muhammad Youssef Abdulazeez of Hixson, Tennessee, or what motivated his rampage.

Federal authorities are looking into the possibility it was an act of terrorism, but say there's no evidence yet that anyone else was involved. The shooter was also killed.

A U.S. official says there was no indication Abdulazeez was on the radar of federal law enforcement before the shootings. The official wasn't authorized to discuss the case and spoke on condition of anonymity.