Aaron Alexis. Photo provided by FBI.

The person believed to be responsible for killing at least 12 people at the Naval Sea Systems Command headquarters in Southeast D.C. has been identified as 34-year-old Aaron Alexis of Texas.

The FBI’s Valerie Parlave, who serves as assistant director in charge of the Washington Field Office, confirmed at a press briefing that Alexis was identified as the Navy Yard shooter. According to the FBI, Alexis was born in Queens, N.Y. and was last known to reside in Fort Worth, Texas.

Alexis died at the scene of the shooting after “multiple engagements” with police. It’s not clear how he got on the base.

Metropolitan Police Department Chief Cathy Lanier said the scene of the shooting “was one of the worst things they have seen” in Washington, D.C. Lanier said they would not comment on evidence recovered. She said police officers are still “actively engaged.”

The Washington Post obtained military personnel records that “show that Alexis spent nearly four years in the Navy as a full-time reservist from May 2007 until he was discharged in January 2011.”

Those Navy officials said they were still researching whether Alexis had been employed as a defense contractor or a civilian employee of the Navy, and were uncertain if he was assigned to work at the Navy Yard.

The Smoking Gun has details about Alexis’ alleged criminal past.

Aaron Alexis, 34, was arrested in September 2010 after his upstairs neighbor called Fort Worth cops to report that a bullet had been shot into her apartment. The round apparently first traveled through her home’s floor and then the ceiling.

The woman, whose name is not listed in a Fort Worth Police Department report, said that she was “terrified” of Alexis, who had previously confronted her about making too much noise. The woman, who was “visibly shaken up” when questioned by police, said that she believed the shooting was “intentional.”

Alexis was also arrested by Seattle police in 2004 for “shooting out the tires of another man’s vehicle in what Alexis later described to detectives as an anger-fueled ‘blackout,'” according to the Seattle Police Department.

At about 8 am that morning, two construction workers had parked their 1986 Honda Accord in the driveway of their worksite, next to a home where Alexis was staying in the Beacon Hill neighborhood.

The victims reported seeing a man, later identified by police as Alexis, walk out of the home next to their worksite, pull a gun from his waistband and fire three shots into the two rear tires of their Honda before he walked slowly back to his home north of the construction site.
Officers responded to the scene but were unable to locate Alexis, and no one answered the door at his home.

When detectives interviewed workers and a manager at the construction site, they told police Alexis had “stared” at construction workers at the job site every day over the last month prior to the shooting. The owner of the construction business told police he believed Alexis was angry over the parking situation around the work site.

Detective notes from the incident indicate they made several attempts to contact Alexis by phone and at his work, but eventually found and arrested him outside of his home on June 3rd.

Police then obtained permission to search the home, found a gun and ammunition in Alexis’ room, and booked him into the King County Jail for malicious mischief.

Following his arrest, Alexis told detectives he perceived he had been “mocked” by construction workers the morning of the incident and said they had “disrespected him.” Alexis also claimed he had an anger-fueled “blackout,” and could not remember firing his gun at the victims’ vehicle until an hour after the incident.

Alexis also told police he was present during “the tragic events of September 11, 2001″ and described “how those events had disturbed him.”

Detectives later spoke with Alexis’ father, who lived in New York at the time, who told police Alexis had anger management problems associated with PTSD, and that Alexis had been an active participant in rescue attempts on September 11th, 2001.