Washington (CNN) -- While no specific reason has been given as to what spurred him to fatally shoot 12 people at Washington's Navy Yard, Aaron Alexis' overall mindset came into sharper focus Tuesday -- including a history of trouble in the Navy and psychological issues.
That past includes a Newport, Rhode Island, incident on August 7, more than five weeks before Alexis was gunned down after single-handedly pulling off America's latest mass shooting.
Describing himself as a Navy contractor, Alexis told police he believed an individual he'd gotten into a verbal spat with had sent three "people to follow him and keep him awake by talking to him and sending vibrations into his body," according to a police report. Alexis said he hadn't seen any of these people, but insisted they'd followed him between three hotels in the area -- the last being a Marriott, where police investigating a harassment complaint encountered him.
There, Alexis told authorities the unseen individuals continued speaking to him through walls and the floor, and that they used "some sort of microwave machine" to send vibrations into his body to keep him awake.
He added, according to the police report, that "he does not have a history of mental illness in his family and that he never had any sort of mental episode." Nonetheless, a police sergeant alerted authorities at Naval Station Newport to Alexis "hearing voices." Reached Tuesday, officials at the base referred CNN to the FBI, which declined to comment.
It's not known if this incident was in any way related to Monday's shooting spree. Still, it and other details offer insights into the shooter and raised questions about whether he could have been stopped.
The Navy moved to discharge Alexis in 2010 due to what two Navy officials described as a "pattern of misconduct," though he ended up leaving the next year on a "general discharge" but maintained his security clearance.
There were also run-ins with police, beyond the Newport incident. In Seattle, for instance, Alexis was arrested in 2004 for shooting out the tires of another man's vehicle in what he later told detectives was an anger-fueled "blackout."
DeKalb County, Georgia, authorities said Tuesday they arrested Alexis in 2008 on a disorderly conduct charge.
And recently, Alexis contacted two Veterans Affairs hospitals around the capital, law enforcement sources told CNN. Two indicated he sought help for sleep-related issues, with another source saying Alexis was "having problems sleeping" and "hearing voices."
Whatever his past, Alexis was a military contractor and was in the Navy's Ready Reserve -- a designation for former military members who don't actively serve in a Reserve unit but who can be called up if the military needs them.
Moreover, he had legitimate credentials to enter the base, Valerie Parlave, assistant director in charge of the FBI's Washington field office, said Tuesday. He also had a secret security clearance valid through 2017, even after leaving the service full-time in 2011, Navy spokesman John Kirby told CNN.
Should he have? Did the military miss opportunities to prevent Alexis from attacking? And how was he able to get a shotgun onto the naval base?
Those are good question, yet none has a definitive answer.
Investigators scour crime scene, hotel and beyond
The historic Navy Yard will remain closed to all but "essential" Navy employees Wednesday, as it was Tuesday. Even then, it's likely to remain abuzz with investigators methodically modeling bullet trajectories and mapping the scene, according to Parlave.
Federal law enforcement sources say authorities recovered three guns from the scene: a shotgun and two handguns.
Two days before the shooting, Alexis spent "a couple hours" shooting at Sharpshooters Small Arms Range in northern Virginia before paying $419 for a Remington 870 shotgun -- after being approved by the federal background check -- and a small amount of ammunition, the store's attorney, J. Michael Slocum, said.
The two handguns, sources say, may have been taken from guards at the naval base. But how Alexis brought the shotgun in, though, remains an open question, with Washington Mayor Vincent Gray speculating he may have concealed it.
Surveillance video shows Alexis walking into the facility, bringing a bag into a bathroom in Building 197 and coming out with the shotgun, a federal law enforcement official said. It's believed Alexis had the disassembled gun inside the bag.
From there, he headed to a perch overlooking the building's atrium and began firing on those below using 00 buckshot shells, each packed with about a dozen pellets capable of causing tremendous damage, according to the same official.
Parlave said Alexis is believed to be solely responsible for Monday's bloodshed that, in addition to those killed, left eight wounded. Three of them suffered gunshot wounds, one of whom was released Tuesday. Another is Washington police Officer Scott Williams credited with killing Alexis. Still, that doesn't mean authorities aren't looking into others who might have helped Alexis, wittingly or unwittingly, or known something about the plot.
Federal investigators on Tuesday collected Alexis' computer and possessions from the hotel where he spent the last few days of his life, in addition to reviewing surveillance and other tools to see whom he interacted with in three weeks around Washington.
Co-workers have portrayed Alexis of having lived the mundane work life of a well-paid tech contractor given daily per diems that allowed a comfortable stay in an expensive city, a senior law enforcement official said.
Authorities are also appealing for the public's help in a probe U.S. Attorney Ronald Machen estimates could take "weeks and months."
"No piece of information is too small," Parlave said.
'Who was this guy?'
A New York City native -- where both his parents, now divorced, still live -- Alexis worked between 2001 and 2003 at the Borough of Manhattan Community College. His supervisor there, Barry Williams, told CNN he never would have expected such a violent outburst, though Alexis would get easily frustrated over minor things and hold grudges.
Years later, Alexis joined the Navy -- which admitted him knowing of his 2004 arrest, having been apparently "satisfied that it did not preclude granting (secret security) clearance," a senior Navy officer said -- as a petty officer working on electrical systems.
But his four years in service weren't all smooth. He was written up for eight instances of misconduct on duty, a U.S. defense official told CNN, including cases of insubordination, disorderly conduct, unauthorized absences and at least one instance of intoxication.
"He wasn't a stellar sailor, we know that," said Rear Adm. Kirby, adding that the misconduct cases were all "relatively minor." "... None of those (offenses) give you an indication he was capable of this sort of brutal, vicious violence."
Rep. Michael McCaul, chairman of the House Homeland Security Committee, said he feels the infractions "were kind of swept under the rug."
"It is real easy to just pass the buck along to another military base or in this instance, a defense contractor," the Texas Republican said Tuesday. "...There are so many red flags that popped up in this case."
Without a civilian conviction, the offenses weren't enough to produce a general discharge; Alexis was granted an honorable discharge in January 2011 instead. In fact, he remained part of the Navy's Ready Reserve up to his death, Navy Secretary Ray Mabus told CNN.
The Experts -- the contracting firm for which Alexis worked for about six months over the past year -- said the last of two background checks it conducted in June on Alexis "revealed no issues other than one minor traffic violation."
Still, did something change more recently? There was the August incident in Rhode Island. And a friend and former housemate, Kristi Suthamtewakul, told CNN's "New Day" she noticed personality changes in Alexis over the last few months, but nothing indicating the potential for such violence.
"Aaron was a very polite, very friendly man," she said.
Among other problems, he had been frustrated about pay and benefits issues after a one-month contracting stint in Japan last year, Suthamtewakul said.
"He got back and he felt very slighted about his benefits at the time," she said. "Financial issues. He wasn't getting paid on time, he wasn't getting paid what he was supposed to be getting paid."
"That's when I first started hearing statements about how he wanted to move out of America," Suthamtewakul said. "He was very frustrated with the government and how, as a veteran, he didn't feel like he was getting treated right or fairly."
Another friend, Texas resident Michael Ritrovato, also said Alexis recently had been frustrated with his employer over pay.
But Ritrovato said his friend never showed signs of aggressiveness or violence, though he played a lot of shooting video games online.
"It's incredible that this is all happening, because he was a very good-natured guy," Ritrovato said. "It seemed like he wanted to get more out of life."
Melinda Downs described Alexis as "very intellectual" and of "sound" mind -- saying if he did hear voices, "he hid it very well." The two spoke as recently as a week ago, at which time Downs said she had no hints of what was to come.
"It is like Dr. Jekyll and Mr. Hyde," she said. "Who was this guy?"
CNN's Michael Pearson and Greg Botelho reported and wrote from Atlanta; Pamela Brown reported from Washington. CNN's Phil Gast, Catherine E. Shoichet. Ed Payne, Chris Lawrence, Barbara Starr, Chris Cuomo, John King, Deborah Feyerick, Evan Perez, Tom Cohen, Dan Merica, Larry Shaughnessy, Brian Todd, Alan Silverleib, Susan Candiotti, Joe Johns, Eliott C. McLaughlin, Joe Sterling, Paul Courson and Ed Lavandera contributed to this report.