Holder pledged the “full resources” of the Justice Department, which includes the FBI, to investigate the shooting by civilian contractor Aaron Alexis.

Alexis bought the shotgun he used in the shooting spree legally from a Virginia gun store after having a background check completed on him.

Several lawmakers have raised questions about Alexis’s mental health, however, citing his arrest in 2011 on weapons charges in Texas while he was serving in the Navy.

Alexis subsequently received an honorable discharge from the Navy. Afterward he checked into two emergency medical centers within a span of five days, receiving a prescription for insomnia at one of them.