CHATTANOOGA, Tenn. (AP) – Secretary of Defense Ash Carter says at a memorial for five servicemen killed in Chattanooga that the shooter’s motivations remain unclear.

Carter said on Saturday that officials may never know “what combination of disturbed mind, violent extremism and hateful ideology was at work.”

The FBI has not been able to determine whether 24-year-old shooter Muhammad Youssef Abdulazeez was “radicalized” before the July 16 attacks and has been treating him as a homegrown violent extremist.

Ash has called for a review of domestic security procedures for military installations to better protect. He said the “hard fist of justice” will reach anyone who tries to harm Americans.

Also attending Saturday’s memorial were Vice President Joe Biden, U.S. Sen. Bob Corker, Tennessee Gov. Bill Haslam and Chattanooga Mayor Andy Berke.