Thirteen people have died after a former Navy reservist opened fire at a naval base in Washington, D.C., Monday morning, killing 12 people and wounding several others before dying in a shootout with police. The gunman has been identified as Aaron Alexis, a 34-year-old who had been arrested at least twice in the past for shooting-related incidents, but who had security clearance to enter the Washington Navy Yard and worked for a military contractor. Alexis was discharged from the Navy Reserve in 2011 following what officials termed a “pattern of misbehavior.” In 2010, he was arrested in Fort Worth, Texas, when his downstairs neighbor complained he had fired a bullet through his floor into her apartment. In 2004, he was arrested in Seattle after firing his pistol into the tires of a car. At the time, Alexis’ father told police his son had participated in the 9/11 rescue effort and had anger issues possibly tied to post-traumatic stress disorder. Alexis told police 9/11 “had disturbed him.” Alexis did technology-related work for a company called the Experts, a subcontractor to Hewlett-Packard. Officials said he purchased at least one of the guns used in Monday’s rampage legally. He was found with three weapons: a shotgun, a semi-automatic pistol and an AR-15 assault rifle. According to the Associated Press, Alexis was being treated at the VA for multiple disorders, including hearing voices. Eight victims have been identified publicly, all of whom are believed to be civilians or contractors. According to Mother Jones, it is the fifth mass shooting in the United States since the massacre at Sandy Hook Elementary School less than a year ago. President Obama responded to the shooting on Monday.

President Obama: “We are confronting yet another mass shooting. And today it happened on a military installation in our nation’s capital. It’s a shooting that targeted our military and civilian personnel. These are men and women who were going to work, doing their job protecting all of us.”