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Officials said Alexis suffered from a sleep disorder and paranoia, and also heard voices in his head.
Aaron Alexis, the gunman who was shot down after opening fire at the Washington Navy Yard Monday, is believed to have acted alone.
The FBI gave additional details about the shooting on Tuesday afternoon, including the fact that Alexis was armed with a shotgun lawfully purchased in Virginia.
Valerie Parlave of the FBI said the authorities did not believe that Alexis was armed with an AR-15 assault rifle. He did, however, gain access to a handgun after the attack began.
The FBI continued to ask the public for any information they may have regarding Alexis.
Law enforcement officials who anonymously spoke to the Associated Press Tuesday said Alexis suffered from several mental health issues, including paranoia and a sleep disorder, for an unknown length of time.
An August 7 police report from Newport, Rhode Island said Alexis called authorities and claimed he was being followed by people who were "talking to him and sending vibrations into his body.”
The former Navy reservist and military subcontractor began treatment at the Veterans Administration in August. Alexis was not declared mentally unfit, which the AP noted would have lost him the secret security clearance he had from his days as a Navy reservist.
In addition to Alexis' mental health issues, he also reportedly had a history of misconduct and violence that resulted in the 34-year-old's discharge from the Navy Reserves in 2011.
In one such incident in 2004, Alexis reportedly shot the tires of a Honda Accord that had been parked next his house in Seattle three times.
In another, he cursed repeatedly after being thrown out of a nightclub. A Navy official told The Washington Post Alexis was cited eight or more times for offenses including skipping work, insubordination and disorderly conduct.
Speaking to police investigating the incident, Alexis' father said his son had "experienced anger management problems that the family believed was associated with post-traumatic stress disorder management issues," according to a police report cited by The Washington Post.
The father also said his son had been “an active participant in rescue attempts of Sept. 11, 2001.”
More from GlobalPost: Suspected Navy Yard shooter Aaron Alexis: A veteran with anger issues (VIDEO)
Others who knew Alexis gave a different account.
"I don't believe he did that," Suthamtewakul, owner of Happy Bowl Thai in White Settlement, Texas, told ABC News, about the Navy Yard shooting. "He can be tough physically, but I don't think he'd kill people."
Suthamtewakul considered Alexis — who he had lived with for three or four years — a "good and close friend." He said, "He had a gun but that doesn't mean he's gonna shoot people. He had a concealed-weapons permit."
Authorities on Tuesday began releasing information about the 12 victims Alexis shot dead on Monday, marking the worst loss-of-life in the District since an airplane crashed in the Potomac River and killed 78 people in 1982.
So far seven victims have been identified. They are Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Kenneth Bernard Proctor, 46; and Vishnu Pandit, 61.
The names of the other five victims were released after their family members were informed. They are Arthur Daniels, 51, Mary Francis Knight, 51, Gerald L. Read, 58, Martin Bodrog, 54, and Richard Michael Ridgell, 52.
DC Mayor Vincent C. Gray said eight more people were injured in the attack, including Metropolitan Police Officer Scott Williams, who is recovering from a gunshot wound and in good condition.
Alexis was killed by security forces on Monday after engaging in a gunbattle.
More from GlobalPost: 13 dead in shooting at US Navy Yard in Washington, DC