Jacquelyn Martin / AP
A member of the Navy checks vehicles at a gate to the Washington Navy Yard, as some employees return, many to retrieve their vehicles, two days after a gunman killed twelve people and was killed himself inside the Navy Yard in Washington, on Wednesday.
The Washington Navy Yard reopened early Thursday, three days after gunman Aaron Alexis killed 12 people and wounded several others in a shooting rampage at the Washington, D.C., base.
The gates at the Navy installation reopened at 6 a.m. Thursday, according to the Associated Press.
Thursday will be a standard work day, excluding Building 197, where the horrific shootings took place, and the base gym, Navy spokeswoman Lt. Cmdr. Sarah Flaherty told the Associated Press. The gym will be used as a staging area for the FBI to probe Monday's massacre, she added.
Authorities say they are still looking for a motive. Since Alexis carried out the attack Monday at the headquarters of the Naval Sea Systems Command, signs have emerged of his troubled history, including a military disciplinary record and reports he suffered from depression and paranoia.
Defense Secretary Chuck Hagel said Wednesday that "obviously there were a lot of red flags" in Alexis' past, including reports that he had complained of insomnia and sought treatment at a VA hospital emergency room, and that the department would look into why they were not picked up.
Alexis was purportedly suffering from insomnia during an Aug. 23 emergency room visit to the VA Medical Center in Providence, R.I., where he was given sleep medication and told to follow up with a doctor, according to the AP.
Five days later, he visited the VA Hospital in Washington, where he said he hadn't been able to sleep due to his work schedule, and again had his medication refilled, according to the wire service.
He seemed "alert and oriented" during those visits and claimed that he didn't feel depressed, anxious or prone to violence, the VA said in a statement provided to legislators Wednesday, according to the AP.
But just two weeks before his emergency room stay, Alexis complained to Rhode Island police that people were communicating to him via the walls and ceilings of his hotel room and transmitting microwave vibrations into his body to keep him from falling asleep.
Newport authorities reported the incident to offers at the base security office, Navy officials said, but there was no follow-through because Alexis didn't appear to pose a threat to himself or others at the time, according to the AP.
President Barack Obama plans to attend a memorial service for the Navy Yard victims Sunday, the White House press secretary said.
The mother of Aaron Alexis, the Washington Navy Yard shooter, said Wednesday that she was heartbroken and sorry for the families of the victims and that she was glad he is "in a place where he can no longer do harm to anyone."
In a brief statement to a reporter in New York, the woman, Cathleen Alexis, said her son "has murdered 12 people and wounded several others."
"His actions have had a profound and everlasting effect on the families of the victims," she said, her voice trembling. "I don't know why he did what he did, and I'll never be able to ask him why. Aaron is now in a place where he can no longer do harm to anyone, and for that I am glad."
She added: "To the families of the victims, I am so, so very sorry that this has happened. My heart is broken."
Earlier in the day, a woman whom Aaron Alexis stayed with in Thailand last year said that he was crazy "in a positive way, like funny," and that she was shocked to learn that he had carried out the massacre at the Navy Yard. The spree ended when Alexis was gunned down by officers.
The woman, Om Suthamtewakul, is the sister of a former roommate of Alexis' in the U.S. She told NBC News in an interview that Alexis stayed with her for a month and a half and showed no sign of anger.
"So I can't really believe how he can shoot those people," she said in Thai. "He looked kind of like, you know, bonkers, crazy, in a positive way, like funny, but, so I really can't believe this."
Suthamtewakul said Alexis liked her country, "loved Thai woman" and wanted to go back. She said that she and Alexis went on outings in Bangkok and elsewhere and that they went to massage parlors in the evening.
She said she never saw him show cruelty.
"Every day he has good mood, laughing," she said, "and one time we went to the market together because he understand Thai and he heard one Thai woman saying rude words about him — but he didn't get angry, he laughed and told the woman, 'I understand what you said.'"
Jeff Black, Tracy Connor, Jason Cumming, Jonathan Dienst, Richard Esposito, Courtney Kube, Charles Hadlock, Peter Jeary, Jim Miklaszewski, Andrew Rafferty, Marian Smith, Daniel Arkin and Ali Weinberg of NBC News contributed to this report.