- "I'll never be able to ask him why" he did what he did, Cathleen Alexis says
- Initial security review to be complete in two weeks, admiral tells congressional panel
- Video shows Alexis going into a bathroom, walking out with a shotgun, official says
- Navy vet, contractor Aaron Alexis killed 12 at Washington's Navy Yard
Share your reaction to the shooting. What, if anything, might have prevented this?
Washington (CNN) -- Washington Navy Yard gunman Aaron Alexis' mother apologized Wednesday for her son's actions, saying 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 a statement recorded by CNN.
"I'm so, so very sorry this has happened. My heart is broken," she said.
Her statement comes two days after Aaron Alexis, a military contractor, shot and killed 12 people at the historic Navy base. The facility was closed again Wednesday to all but a few employees as authorities worked to piece together what triggered the shooting.
A police officer runs near the scene of the shooting rampage at the Washington Navy Yard on Monday, September 16. Authorities said at least 12 people -- and the suspect -- were killed in the shooting. 
A U.S. Park Police officer stands guard near the scene of the shooting. 
Military security observes the scene from a nearby rooftop. 
The White House flies the U.S. flag at half-staff following the shooting. 
A police boat patrols the waters at the Washington Navy Yard. 
Late Monday afternoon the FBI identified Aaron Alexis, a 34-year-old military contractor from Texas, as the dead suspect involved in the shooting. 
Workers who had been sheltered during a lockdown exit the Navy Yard. 
People step off buses at Parking Lot B of Nationals Park, which was set up as a gathering point for family members of Navy Yard employees in the wake of the shooting. 
A man and woman embrace at the gathering point inside Nationals Park. 
A man walks with a woman after they were reunited at Nationals Park. 
A woman reunites with her child inside Nationals Park. 
Washington Mayor Vincent Gray and Washington Police Chief Cathy Lanier speak to the media near the Washington Navy Yard. 
Police officers walk on a rooftop at the Washington Navy Yard. 
A member of the military stands guard at the scene of the shooting. 
A woman watches from an office building as police respond to the shooting. 
Police officers leave the scene after responding to the shooting. 
A police officer and police canine inspect vehicles in a parking lot outside the Washington Navy Yard. 
A Homeland Security officer tells a woman where to park and wait for her husband, who works in the Washington Navy Yard. 
A police helicopter hovers above snipers on the roof of a building at the Washington Navy Yard. 
Police stand guard on M Street in Washington. 
Police boats patrol the Anacostia River off the Washington Navy Yard complex. 
A U.S. Capitol Police officer keeps watch on the East Plaza of the Capitol in Washington. 
Troops stand guard in front of a parking garage near the Washington Navy Yard. 
Police vehicles move toward the Navy Yard. 
Armed agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives work on the 11th Street Bridge adjacent to the Washington Navy Yard. 
Emergency vehicles fill the streets around the Washington Navy Yard. 
A helicopter lifts a person off a roof at the Navy Yard. 
A police officer carries an automatic rifle at the Washington Navy Yard. 
Police officers adjust their gear. 
Police block off a road leading to the Washington Navy Yard. Mass shooting at D.C. Navy Yard
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Photos: Mass shooting at D.C. Navy Yard 
Navy Yard shooter's security clearance 
Conversation with the Navy Yard shooter 
Hagel: We'll fix security gaps So far, their investigation has uncovered little to explain the rampage, a senior law enforcement source told CNN.
Federal investigators have collected Alexis' computer and other possessions from the hotel where he spent his last days, the source said. They also worked to talk to people he'd met since coming to Washington three weeks before Monday's shooting spree at the Navy Yard.
But nothing so far has pointed to a specific motive for the killings, a second law enforcement source told CNN.
There are potential clues: In August, he told police in Newport, Rhode Island, that he was hearing voices and was convinced that someone was using a "microwave machine" to send vibrations into his body to keep him awake, according to an incident report.
Read the police report
He'd sought help from Veterans Affairs hospitals around the capital, law enforcement sources told CNN. One said he was hearing voices and having problems sleeping.
His checkered history as a Navy sailor and run-ins with police also seemed to offer clues into a sometimes troubled personality.
But even that, Navy Rear Adm. John Kirby said, offered no hint that Alexis was dangerous.
"Looking at the offenses while he was in the Navy, the offenses while he was in uniform, none of those give you an indication that he was capable of this sort of brutal, vicious violence," Kirby told CNN's Wolf Blitzer.
Meanwhile, hospital officials said one of the three hospitalized victims of the attack had been released. The woman had been injured by a bullet that struck behind her ear, doctors previously said. Two other people -- a civilian and a Washington police officer -- remain hospitalized in fair condition, doctors say. The officer, Scott Williams, is believed to have fired the shot that killed Alexis, ending his rampage.
If you have information regarding Aaron Alexis or the Navy Yard shooting, call 1-800-CALL-FBI or
visit the FBI's website.
Who were the victims?
Investigators scour crime scene, hotel and beyond
Authorities say Alexis entered the Navy Yard on Monday morning using a valid identification card. He went into Building 197, where the killings all took place, carrying a bag that may have contained a disassembled shotgun, a federal law enforcement source said. Surveillance video shows him walking into a bathroom in the building and coming out with the shotgun, the official said.
Two days before Monday's 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. Alexis passed a federal background check for the purchase, Slocum said.
Witness: 'He aimed his gun at us' and fired
Shooter's mother: 'I am so, so very sorry' 
Who was Aaron Alexis? 
Tragedy at the Washington Navy Yard 
Obama: Shooting was a 'cowardly act' While authorities have provided few details of what happened inside Building 197, witnesses reported seeing what appeared to be a determined gunman taking aim at seemingly random victims. Twelve people died and eight were wounded. The rampage ended when Alexis was shot to death.
Federal law enforcement sources say authorities recovered three guns from the scene: a shotgun and two handguns. The two handguns, sources say, may have been taken from guards at the naval base.
FBI teams remained at the base Wednesday amid a nationwide investigation that U.S. Attorney Ronald Machen said could take "weeks and months."
Authorities also pleaded for the public's help.
"No piece of information is too small," Valerie Parlave, assistant director in charge of the FBI's Washington Field Office, said Tuesday.
Who is Navy Yard gunman Aaron Alexis?
Hearing voices
While Alexis' employer, an information technology contractor called The Experts, said it had no reports of trouble with him from Navy bases where he had worked over the summer, a picture was building of an increasingly troubled mind.
On August 7, Alexis told police in Newport, Rhode Island, that he believed he was being followed by three people who had been dispatched by someone with whom he'd quarreled, a police report said.
He said they had been sent to "follow him and keep him awake by talking to him and sending vibrations into his body," according to the 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 stopped to talk with 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.
Shooting at Washington Navy Yard Shooting at Washington Navy Yard 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.
That lines up with law enforcement reports that Alexis had sought treatment at two capital-area Veterans Affairs hospitals. One law enforcement source told CNN that Alexis told hospital officials he was hearing voices and having problems sleeping.
Benita Bell met Alexis last week at the Residence Inn where he was staying before the shootings.
On the Tuesday before the shootings, he seemed "engaging, present, connected," Bell told CNN. On Wednesday, he seemed markedly different -- stressed and hurried, she said.
"He said he was extremely tired, exhausted," Bell said.
Earlier incidents
The Navy moved to discharge Alexis in 2010 due to what two Navy officials described as a "pattern of misconduct." Those incidents involved insubordination, disorderly conduct, unauthorized absences from work and at least one instance of drunkenness, a U.S. defense official told CNN.
Because of a lack of evidence, authorities were unable to get a general discharge that might have had an impact on his ability to get civilian work, the official said. Instead, he was given an honorable discharge and later hired as a civilian military contractor after passing security reviews.
There also were run-ins with police, beyond the Newport incident. Seattle police arrested Alexis in 2004 on accusations that he shot out the tires of another man's vehicle in what he later told detectives was an anger-fueled "blackout." He was arrested in 2008 in DeKalb County, Georgia, on a disorderly conduct charge.
Friends said Alexis didn't seem capable of such violence.
"Aaron was a very polite, very friendly man," said Kristi Suthamtewakul, a friend and former housemate.
But he was 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, said Alexis never showed signs of aggressiveness or violence, although he played a lot of shooting video games.
"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?"
Security questions
Vetting military contractors: How did gunman get in?
The revelations about Alexis' past have led to questions about whether he should have retained his security clearance after leaving the Navy or been allowed to obtain a job working on military bases.
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."
Rep. Michael McCaul, chairman of the House Homeland Security Committee, said Tuesday 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."
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.
Reddit shuts down community seeking shooter
CNN's Michael Pearson and Ed Payne reported and wrote from Atlanta; Pamela Brown reported from Washington. CNN's Phil Gast, Catherine E. Shoichet. Greg Botelho, 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.
