(Last updated Sept. 17 at 3:24 p.m.)

Thirteen people are dead, including the shooter, and more are injured after a shooting at the Washington Navy Yard Monday morning in D.C.

The shooting is the deadliest on a military installation in the U.S. since the tragedy at Fort Hood, Texas, in 2009.

The FBI identified the shooter, who was killed during the event, as 34-year-old Aaron Alexis of Texas.

Police Chief Cathy Lanier said there is still no known motive for the shooting.

Law enforcement officials told The Associated Press Alexis had been suffering a host of serious mental issues, including paranoia and a sleep disorder. He also had been hearing voices in his head. Alexis had been receiving treatment since August.

Alexis used a shotgun he had purchased and two handguns that he took from a law enforcement officer during his attack at the Navy Yard, but not an AR-15 assault rifle, as officials previously said. Two law enforcement officials told The Associated Press that an AR-15 was found at the scene. It was not immediately clear whether the rifle belonged to a law enforcement or security officer responding to the gun battle.

Authorities are now convinced Alexis acted alone. Police had originally been on the lookout for two potential additional suspects, but those people were later cleared by police.

“We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today,” Lanier said.

The D.C. Police Department released the names of all 12 victims Tuesday: Michael Arnold, 59; Martin Bodrog, 54; Arthur Daniels, 51; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Mary Francis Knight, 51; Kenneth Bernard Proctor, 46; Vishnu Pandit, 61; Gerald L. Read, 58; Richard Michael Ridgell, 52.

A hospital official says a police officer and two civilians wounded in the Washington Navy Yard shooting are doing well.

Dr. Janis Orlowski, MedStar Washington Hospital Center’s chief medical officer, said Tuesday morning that the police officer, who was shot in the legs, and a woman who was shot in the shoulder are in fair condition. She says a young woman whose skull was grazed by a bullet is in good condition and asked Monday if she could go home.

The FBI has taken the lead in the investigation.

Valerie Parlave, the FBI’s assistant director in charge, said Tuesday that Alexis arrived in D.C. around August 25 and had been staying at various hotels in the area. On Sept. 7, Alexis checked into the Residence Inn in Southwest D.C., the most recent hotel he is known to have stayed at.

People with any information about the shooting or Alexis are encouraged to call 1-800-CALL-FBI.

The Washington Navy Yard remains closed Tuesday, except for mission-essential personnel.

The Department of Transportation located at 1200 New Jersey Ave, SE, and the Coast Guard headquarters at 2100 2nd St., SW, were both locked down Monday due to their proximity to the incident. Bolling Air Force Base and the Naval Research Lab were also closed out of precaution.

Andrews Air Force Base and Joint Base Myer-Henderson Hall remained open during the incident. Fort McNair also remained open, but visitors were only permitted to enter and exit through the 2nd St. gate.

(Federal News Radio’s Jack Moore contributed to this report)

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