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.