WASHINGTON - Investigators believe gunman Aaron Alexis acted alone yesterday during a shooting rampage at the Washington Navy Yard that left 12 people dead and several others injured.

Authorities say the 34-year-old was killed during a shootout with police.

They say Alexis was a former Navy reservist working as a defense contractor and was able to enter the facility with credentials.

According to investigators, Alexis also had a criminal history record and was arrested twice before for shooting his gun without reason.

The military says he was discharged from the Navy after a "pattern of misconduct."