NASHVILLE, Tenn. — The public may never know what motivated a 24-year-old Chattanooga man to kill four Marines and a sailor in an attack on Chattanooga's U.S. Naval and Marine Reserve Center last July. Investigators have said Muhammad Youssef Abdulazeez was a homegrown violent extremist but have not offered more details about what motivated the attack that began at a military recruiting center and ended when Abdulazeez was shot to death by police who followed him to the reserve center. "We're still trying to make sure we understand Abdulazeez, his motivations and associations, in a really good way," FBI Director...