(AP) — The Navy says a gunman who opened fire inside a building at the Washington Navy Yard was a full-time reservist from 2007 to 2011.

The Navy said in a release Monday that 34-year-old Aaron Alexis, of Texas, left the Navy on Jan. 31, 2011, as a petty officer 3rd class. It’s not immediately clear why he left.

Alexis had been working for the fleet logistics support squadron No. 46, in Fort Worth, Texas. The Navy says his home of record was New York City.

A Navy Official speaking on condition of anonymity said Alexis is believed to have gotten into the Navy Yard by using someone else’s identification card. It is not yet clear if that individual was an accomplice or if that person’s ID card was stolen.

Hundreds of law enforcement officers are looking for a possible second suspect.

Alexis was one of 13 people killed during the rampage.