Washington Navy Yard gunman Aaron Alexis' mother apologized Wednesday for her son's actions and said she was glad that he is "now in a place where he can no longer do harm to anyone."
"I don't know why he did what he did, and I'll never be able to ask him why, Cathleen Alexis said in an audio statement.
"I'm so, so very sorry this has happened. My heart is broken," she said.
Her statement comes two days after Alexis killed 12 people at the historic Navy base.
The facility will remain closed to all but "essential" employees again Wednesday as authorities piece together what triggered a military contractor to fatally shoot 12 people there.
The investigation continues as the families of Connecticut's Sandy Hook Elementary School shooting victims visit Capitol Hill to mark nine months since the tragic mass shooting and call on Congress to act on legislation to reduce gun violence now.
What made him do it?
While no specific reason has been given on why Aaron Alexis went on a murderous rampage at Navy Yard, his 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.
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. He said 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 related to Monday's shooting spree. Still, it and other details offer insights into the shooter and raise 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."
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? There are no definitive answers.
Investigators scour crime scene, hotel and beyond
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 and a small amount of ammunition, said the store's attorney, J. Michael Slocum.
He was approved after a federal background check, he said.
The two handguns, sources say, may have been taken from guards at the naval base. But how Alexis brought the shotgun in remains an open question, with Washington Mayor Vincent Gray speculating that he 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 the injured suffered gunshot wounds, one of whom was released Tuesday. Another is Washington police Officer Scott Williams, who is 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. They also reviewed surveillance and other tools to see whom he interacted with in three weeks around Washington.
Co-workers have portrayed Alexis as 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 an investigation that 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 native of New York City -- 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 about minor things and hold grudges.
Years later, Alexis joined the Navy, rising to the rank of petty officer and 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, the Navy spokesman, 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."
The revelations led to questions about whether Alexis should have retained his security clearance or been able to obtain a job working on military bases. Defense officials have ordered a review of security measures at U.S. installations worldwide.
On Wednesday, Adm. Jonathan Greenert, the Navy chief of operations, told a congressional committee that an initial review of security and access controls at Navy installations around the world would be completed within two weeks.
Rep. Michael McCaul, chairman of the House Homeland Security Committee, said earlier that he felt Alexis' 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. 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" that 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."
Friend 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.