The Navy was told by authorities in Rhode Island last month of an incident in which Aaron Alexis complained that he was hearing voices, according to a police report on the incident. It is unclear whether the Navy followed up.

Alexis was agitated when police paid him a visit at the Marriott Hotel he had checked into in Newport, R.I.

Local law enforcement had received a harassment complaint from someone in the hotel, and when they tracked down the source of the complaint -- Alexis, the gunman in the Washington Navy Yard massacre on Monday that killed 12 and wounded 8 more -- the police wrote in their report of the incident that he complained about an "unknown party" of people he had never seen. Alexis said these people had been keeping him awake by "sending vibrations into his body" by "using some sort of microwave machine."

• The Navy Yard shooting, as it happened

While the motive in the Navy Yard massacre is still unclear, a pattern of agitated and unsteady behavior has emerged in the recent history of the suspected shooter. Officials say they're fairly certain the 34-year-old former Navy reservist had no connections to terrorism, but instead had a long history of mental health issues.

Alexis told Newport police in August the unknown party following him around had been sending voices through the walls and ceiling. In their report, police said they tried to find out what the voices were saying, but Alexis wouldn't elaborate. Before parting, police claim Alexis said "he does not have a history of mental illness in his family and that he has never had any sort of mental episode."

Police also said in the report that Alexis told them he had checked out of two other hotels to try to escape the voices before checking into the Marriott.

Newport Police Sgt. Frank Rosa says in the report that he informed the Navy of the Marriott incident. "Based on the Naval base implications," Rosa wrote, "and the claim that the involved subject ... was 'hearing voices,' I made contact with on duty Naval Station Police. I advised of the report and the claims." Rose added that Navy authorities said they would follow up to determine if Alexis was indeed a Navy base contractor.

A look at Alexis' prior arrest record indicates that he has a history of mental health issues.

On June 3, 2004, Alexis was arrested in Seattle for shooting out the tires of another man's car on May 6 in what Alexis told police was an anger-fueled "blackout." It appeared Alexis may have been angry with construction workers who were using parking spaces in his neighborhood.

On Sept. 4, 2010, Alexis was arrested for allegedly firing a gun inside his Texas apartment because he had been angry about his downstairs neighbor making too much noise. At the time he was a Navy reservist stationed in Fort Worth.

Officials say Alexis also had bouts of insubordination, disorderly conduct and being absent from work without authorization during his four years as a full-time reservist. He frequently complained about discrimination, and officials say his behavior led to an early but honorable discharge in 2011.

When he left the Navy reserves, he managed to keep his security clearance upon becoming a military contractor because his incidents with police never rose to the level of raising an alarm about him being mentally unfit. Additionally, his arrests never gave cause to bar him from buying a firearm, including the shotgun officials say he bought recently in Virginia to carry out the Navy Yard massacre.

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At the time of the rampage, Alexis was an employee with a Defense Department subcontractor on a Navy-Marine Corps computer project, authorities told the Associated Press. Alexis had access to the Navy Yard via a valid pass.

The roots of Alexis' issues are still being examined, but after his 2004 arrest in Seattle, both Alexis' father and Alexis himself told police he had been present during "the tragic events of September 11, 2001" and that they had disturbed him. His father told police he suffered from PTSD and that his mental issues stretch back at least a decade.

After the Newport, R.I., run-in with police August, Alexissought treatment for mental health issues from the Department of Veterans Affairs, officials say.

Until recently, Alexis lived in Fort Worth at the home of Melinda Downs, reports CBS News correspondent Bob Orr. She said Alexis suffered from PTSD and insomnia. She told reporters he loved the military and showed no signs of violence.

"It's like Dr. Jekyll and Mr. Hyde, who was this guy?" Downs said. "The guy that I knew was so honorable. It breaks my heart."