Law enforcement officials say the gunman who attacked the Navy Yard on Monday had been suffering from “serious mental issues,” according to The Associated Press.

Aaron Alexis, 34, who police say died Monday after killing a dozen people, was suffering from paranoia and a sleep disorder, the AP reports. Alexis had been hearing voices in his head, according to police.

Since August, he was being treated for his mental problems by the Department of Veterans Affairs, the report says. 

Officials spoke on the condition of anonymity because the criminal investigation is ongoing. 

Neither the FBI, which has the lead on the case, nor the VA would confirm the report’s details to The Hill Tuesday morning because they said the investigation is still fluid. 

Alexis had been arrested in 2004, the Seattle Police Department said, after shooting the tires of someone else’s car in what Alexis described as an “anger-fueled shooting.” 

During that investigation, Alexis’s father, who was living in New York, told police his son had “anger management problems associated with post-traumatic stress disorder.” He also said his son aided in the rescue efforts on 9/11, an event which Alexis said had disturbed him. 

The FBI is seeking more information about Alexis to examine his background and any possible motive. 

Alexis was a military contractor, the bureau’s assistant director of the Washington field office said, and had official access to the Naval Sea Systems Command Headquarters where the shooting took place. 

Multiple reports said he was an employee of a Hewlett-Packard sub-contractor called The Experts. 

The Experts’ Facebook page expressed condolences Monday, and said, “We are actively cooperating with the FBI and other authorities in relation to the investigation on the suspect.”

From 2007 to 2011, Alexis served in the Navy and earned the rank of petty officer 3rd class. During much of that time, he lived in Fort Worth, Texas. He did not serve overseas. 

The Fort Worth Star-Telegram reported Monday Alexis was also arrested in 2010 for shooting a gun at his home. He told police at the time his weapon discharged as he was cleaning it. 

Alexis is originally from New York City. 

Only seven of the 12 shooting victims’ names had been released Tuesday morning by Metropolitan Police because families of the remaining five still need to be notified.