Washington (CNN) -- Details are emerging about Aaron Alexis and his chaotic morning of violence that left 12 people dead at the Washington Navy Yard on Monday. But more questions remain. Here are the latest developments:
• Vice President Joe Biden arrived Wednesday at MedStar Washington Hospital Center to visit those injured in the shooting.
• Damage inside Building 197, the site of the shooting, 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.
• Alexis made unexplained 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.
• An autopsy on the suspected shooter 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.
• 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 VA 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.
• 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.
Mother of shooter apologizes
• Cathleen Alexis, the mother of Washington Navy Yard shooter Aaron Alexis, issued an audio statement on Wednesday.
• "Our son, Aaron Alexis, has murdered 12 people and wounded several others. His actions have had a profound and everlasting effect on the families of the victims. 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. To the families of the victims, I'm so, so very sorry this has happened. My heart is broken."
Open to essential personnel
• The Navy Yard crime scene continues to be active and is open only to essential personnel, spokesman Ed Zeigler said.
• Access to Building 197 is still prohibited.
Sequence of events
• When Alexis entered the building, he had a small bag believed to have contained a disassembled Remington 870 shotgun, a federal law enforcement official says.
• Alexis was seen on surveillance video ducking into a bathroom with the small bag and leaving with the shotgun.
• He also had "00" buckshot shells, each packed with about a dozen pellets capable of causing tremendous damage.
• Denise Robinson, who works in Building 197, told CNN's "New Day" on Wednesday that she saw the shooter and remembers his "cold stare."
"He looked very calm and composed. He didn't look like a person that was angry or doing that type of event," she said.
Security clearance questions
• Defense Secretary Chuck Hagel announced Wednesday a review of all physical and access procedures at U.S. military installations worldwide as first reported Tuesday by CNN's Barbara Starr.
• Deputy Defense Secretary Ashton Carter has been directed to lead a review of Defense Department procedures for granting and renewing security clearances, including those of contractors, Hagel said. Hagel said that review would be conducted in coordination with other government agencies. He also announced the formation of an outside panel to look at Defense Department security and clearance procedures.
• "Everything possible" will be done to prevent shootings like the one at the Navy Yard "from ever happening again," Hagel said. "Our people deserve safe and secure workplaces."
• A senior Navy officer told CNN's Jake Tapper that Navy officials knew about Alexis' 2004 arrest for shooting out the tires of a car, but they decided to grant him security clearance anyway in 2007.
• The officer says the investigators knew of the incident and interviewed Alexis but decided it did not preclude granting the clearance. He "should have been screened out early in his enlistment," one expert says.
• Mert Miller, an Office of Personnel Management official, said the agency conducted a "background security clearance investigation for Aaron Alexis in 2007, and the Department of Defense adjudicated his file and granted his security clearance in 2008."
• The agency is working with the Office of Management and Budget and the director of national intelligence "to review the oversight, nature and implementation of security and suitability standards for federal employees and contractors," Miller said Tuesday.
• "In general, background security clearance investigations include information about an individual's criminal history, including criminal records, and that information would be passed on to the adjudicating agency," said Miller, associate director of the Office of Personnel Management's Federal Investigative Services.
• Some question how Alexis, who had some run-ins with the law, was allowed access to a military installation. The Navy ordered a review of security and access controls at Navy installations around the world. An initial review would be completed within two weeks, Adm. Jonathan Greenert, the Navy chief of operations, told a congressional committee Wednesday.
• Sen. Susan Collins, R-Maine, told "New Day" on Wednesday that "serious questions" need to be posed "about whether contractors are taking shortcuts that have led to people with criminal records, with serious mental illness, or who are authorized unsuited for security clearances, nevertheless being granted them, and for a period as long as five to 10 years."
• Collins said it is "simply inexcusable" for the Navy "to have received a call that indicates that an individual with unfettered access to a Navy base clearly is suffering from a serious mental illness and not immediately revoke his security clearance until they can assure that he does not pose a threat to others and himself."
• "It's incumbent," Collins said, "to work together to get to the bottom of this crisis and ensure that it is fixed."
"One step that can be taken immediately is ensuring that criminal databases and the terrorist watch lists are always consulted, and that there's some sort of continuous monitoring that would pick up problems rather than waiting as long as five or 10 years to review security clearances."
Who are the victims?
• They were civilians and contractors, just starting their day at a massive military compound that's normally a bastion of safety. But for reasons that may never be known, Alexis, a former Navy reservist, cut their lives short.
• The family of Sylvia R. Frasier, one of the 12 people killed in the Washington Navy Yard shooting, said in a statement that she had been "a friend to everyone she met and a stranger to no one." "The only hurt she caused us was the void that was left when she was snatched up prematurely and gone too soon. She will be sorely missed."