(AP) — The FBI says there is no indication that the Navy Yard shooter targeted any specific individuals when he opened fire inside a building, killing 12 people.

Valerie Parlave, head of the FBI’s field office in Washington, said Wednesday during a news conference that investigators are continuing to explore the background and motivations of 34-year-old Aaron Alexis. However, she says Alexis had a well-documented history of mental health issues.

Parlave says Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves. Alexis had written “my ELF weapon” on a shotgun he used in the rampage.

Alexis, a former Navy reservist and IT contractor, was killed in a shootout with police.