Washington (CNN) -- John Weaver remembers the exact moment Washington Navy Yard gunman Aaron Alexis shot his friend.
"He aimed and shot directly at her," he told CNN's Anderson Cooper during an interview that aired Wednesday night.
The bullet struck behind her ear.
"She told me that the force of the blast was so strong it drove her into the ground, and now she has a black eye and bits of her scalp are scattered all over her cubicle," Weaver said. "It was that close to her being killed."
His comments came two days after Alexis, a military contractor, shot and killed 12 people at the historic Navy base. Eight people were injured.
"I got lucky. It was my birthday, and I consider myself the second luckiest person on that day because my friend was the first luckiest person. But all those other people, they did not deserve that death," said Weaver, who knew six of the 12 people killed.
Federal investigators are working to piece together exactly what might have triggered Monday's shooting spree.
They have collected Alexis' computer and other possessions from the hotel where he spent his last days, a senior law enforcement source said. They have also worked to talk to people he'd met since coming to Washington three weeks before the rampage.
Alexis made etchings into the shotgun used in the attack, according to a federal law enforcement official. The etchings read "better off this way" and "my elf weapon," the source said. Investigators don't know what the engravings refer to.
So far, nothing has pointed to a specific motive for the killings, a second law enforcement source told CNN.
There are potential clues: In August, Alexis 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.
He had sought help from Veterans Affairs hospitals.
Alexis received treatment on August 23 when he visited the emergency room at the VA Medical Center in Providence, Rhode Island, according to a statement from the Department of Veterans Affairs. He had complained of insomnia and was given a small amount of medication to help him sleep. On August 28, he went to the VA Medical Center in Washington to request a refill.
In both instances, the statement said, Alexis was asked whether he was struggling with anxiety or depression, or had thoughts about harming himself or others. He reportedly said he did not.
His checkered history as a Navy sailor and run-ins with police also seemed to offer evidence of 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.
Aaron Alexis' mother: 'My heart is broken'
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.
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 said. The officer, Scott Williams, is believed to have fired the shot that killed Alexis, ending his rampage.
Vice President Joe Biden arrived Wednesday at MedStar Washington Hospital Center to visit those injured in the shooting.
The White House announced President Barack Obama will attend a memorial service for the victims at the Navy Yard on Sunday.
"The president will want to mourn the loss of these innocent victims and share in the nation's pain in the aftermath of another senseless mass shooting," White House spokesman Jay Carney said.
Also Wednesday, Navy officials allowed employees back on the base to pick up personal belongings, Capt. Monte Ulmer, commander of Naval Support Activity Washington, told CNN affiliate WJLA. It was scheduled to resume normal operations on Thursday, he said.
The facility remained otherwise closed except for a handful of mission-essential workers.
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 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.
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.
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."
The damage inside Building 197 is significant, according to two Navy officials. There are bullet holes and broken doors and windows. There are also paint markings from law enforcement as they cleared certain areas.
"There is blood everywhere. There is damage everywhere," said one of the officials.
Tactical officers with the U.S. Capitol Police attempting to help stop the deadly rampage Monday were told by a watch commander to return to their position at the Capitol, CNN has learned.
In a statement, the agency said its officers had "offered and provided mutual support and assistance," adding that Chief Kim Dine had opened an investigation.
CNN affiliate WUSA first reported on the controversy.
The station cited an unnamed law enforcement source as saying the team was less than 30 seconds from the base gate when the first call went out alerting police of the shooting.
The team was preparing to enter the facility in a bid to stop the gunman when the commander called them back to the Capitol, the station reported without attribution.
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.
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.
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.
Alexis' autopsy is expected to be completed by the end of the day Wednesday, according to a spokeswoman for the Office of the Chief Medical Examiner in Washington.
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."
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?"
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. "...There are so many red flags that popped up in this case."
On Wednesday, Defense Secretary Chuck Hagel announced the agency would examine security at bases worldwide. He also said his deputy, Ashton Carter, will look at procedures for granting and renewing security clearances.
"There are many questions that are going to be asked, need to be asked, many reviews, and the intensity of those reviews have to go down to every aspect, the security of the physical premises, security clearance, standards of that security clearance, are they strong enough? Why do we do certain things the way we do?" Hagel said. "We need answers, and we will find those."
CNN's Michael Pearson and Ed Payne reported and wrote from Atlanta; Pamela Brown reported from Washington. CNN's Dana Ford, 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, Yon Pomrenze and Ed Lavandera contributed to this report.