Red flags, unexplained etchings on a shotgun, claims of poor police response. It's been three days since Aaron Alexis went on a shooting rampage at the Navy Yard in Washington. And each new day since has brought new revelations about his past, the attack and the subsequent inquiry.

Talk centered this week on how Alexis could maintain his security clearance and access to military contracting jobs despite a checkered history in the Navy and with civilian police.

Defense Secretary Chuck Hagel announced Wednesday that the Defense Department would stage a comprehensive review of security practices at military bases worldwide. And late Wednesday night, the Navy secretary said he had ordered three reviews of security clearances, including one into the granting of Alexis' clearance.

Here's a recap of the latest developments, meant to bring you up to speed quickly.

Photos: Photos: Nation mourns Navy Yard victims Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – A woman who said she works at the Washington Navy Yard looks at a memorial to the shooting victims on Wednesday, September 18. Authorities said 12 people -- plus the gunman -- were killed in the shooting on Monday, September 16. View photos from the scene of the rampage. Hide Caption 1 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – A handwritten note is taped to a post across the street from the Washington Navy Yard on September 18. Hide Caption 2 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – A man puts a bouquet at the anchor outside the Washington Navy Yard on Tuesday, September 17. Hide Caption 3 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – A flag flies at half-staff behind Valerie Parlave of the FBI and Washington Police Chief Cathy Lanier during a press conference on September 17 outside the FBI's field office in Washington. Hide Caption 4 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Defense Secretary Chuck Hagel, right, and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, second from right, present a wreath at the Navy Memorial in Washington on September 17. Hide Caption 5 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Washington Nationals relief pitcher Ian Krol wears a Navy hat in the dugout of Nationals Park in Washington before a game against the Atlanta Braves on September 17. Hide Caption 6 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – The Washington Nationals have a moment of silence for the shooting victims before their game on September 17. Hide Caption 7 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – U.S. flags surrounding the Washington Monument fly at half-staff on September 17. Hide Caption 8 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – From left, Brittany Carter, Jibri Johnson and Bryan Beard attend a small candlelight vigil at Freedom Plaza in Washington on September 16. Hide Caption 9 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Brittany Carter wipes tears away at the September 16 vigil. Hide Caption 10 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Dave Gray and his wife, Brittany, of Salt Lake City hold candles in remembrance of people affected by gun violence. Hide Caption 11 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – The U.S. flag above the Treasury Department flies at half-staff on September 16. Hide Caption 12 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Gordon Morris, left, and wife Laura watch as flags are lowered to half-staff during the September 16 vigil at Freedom Plaza. Hide Caption 13 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Eddie Weingart of Project End Gun Violence speaks to the media on September 16. Hide Caption 14 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – Members of the media cover the vigil on September 16. Hide Caption 15 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – The U.S. flag flies at half-staff above the White House the day of the deadly shooting. Hide Caption 16 of 17 Photos: Photos: Nation mourns Navy Yard victims Nation mourns Navy Yard shooting victims – The Rev. Andrew Royals offers communion to Vinh Tran during a Catholic Mass at the St. Vincent De Paul Church on September 16. A prayer for the shooting victims was held at the church, which is just blocks away from the site of the violence. Hide Caption 17 of 17

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GETTING IN

Valid identification: Alexis had just started a contracting job on the base, so he was able to gain access to Building 197 like any other worker. He walked in, a law enforcement source said, with a bag slung in over his shoulder, passing through security with other employees arriving for work. While red flags come to light after the shooting, Navy officials say none of the details would have been enough to deny him clearance.

Background check: Government contractor USIS said Thursday that it performed Alexis' 2007 background check for the Office of Personnel Management. USIS is the same firm that performed former National Security Agency contractor Edward Snowden's background check for OPM that granted him a security clearance. The firm is under investigation.

Disassembled shotgun: The bag Alexis was carrying apparently contained a disassembled Remington 870 shotgun, a law enforcement official said. Alexis is seen on surveillance video ducking into a bathroom and leaving with the shotgun.

THE ATTACK

30 minutes: Authorities believe Alexis came out of the bathroom and immediately began shooting at random, FBI Director James Comey said Thursday. Investigators believe he then went floor to floor shooting at workers, Comey said. Authorities and witnesses had previously said it appeared Alexis shot mostly from the building's atrium. According to law enforcement officials, he shot a guard on the first floor and took his handgun. It took more than 30 minutes for police to hunt down and kill Alexis, D.C. Metropolitan Police Department Chief Cathy Lanier said.

Unexplained etchings: Alexis etched phrases into the shotgun, a source said. But investigators don't know what to make of it. They read: "Better off this way" and "My elf weapon."

THE RESPONSE

Stand down: A government official told CNN that when the first radio call came in about a shooting at the Navy Yard, highly trained tactical U.S. Capitol Police officers headed to the base but were told by a watch commander to stand down. The chief has now ordered an "independent fact review." D.C. Police Chief Lanier told CNN's Wolf Blitzer on Thursday that she was not aware of anyone being turned away. "I don't think you could have had a faster response," she said.

Undermanned: The Navy Yard also has its own police force. But there were only seven officers working Monday, a police officer and a union official said.

THE SCENE

Blood everywhere: Navy officials say damage inside Building 197 is so extensive, it may take weeks to repair and reopen it. "There is blood everywhere," one official said. "There is damage everywhere."

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THE VICTIMS

Steady recovery: One of the three hospitalized shooting victims has been released. The woman was struck behind the ear. Two others -- a civilian and a police officer -- remain in fair condition. The officer, Scott Williams, is in "great spirits," said Chief Lanier. "I do think he's going to be OK, and recover fully, but he's still having some pretty serious complications," she said.

THE MOTIVE

Still a mystery: Investigators have talked to Alexis' friends and family. They've pored over his computers and other possessions. But, say law enforcement sources, nothing points to a specific motive for the rampage. "I'll reserve judgment on that until all of the work of the FBI is done," Lanier told CNN's Blitzer. "There's still a lot more to do."

Hearing voices: 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. Police notified the Navy. The Navy isn't commenting.

Strange behavior: Also in August, Alexis apparently had an altercation with a family at an airport in Virginia. He thought a woman was laughing at him and became angry. "He started getting a little belligerent, and started using profanity, and going off," said Glynda Boyd. Her brother approached someone to call security, which calmed him down. But Alexis left an unsettling impression. "It was weird. That never happened to us before like that. We had just come off a great vacation," said Michael Boyd.

Trouble sleeping: That month, the VA Medical Center in Providence gave him medicine to help him sleep. A few days later, he went to a VA medical center in Washington to request a refill. Both times, he was asked if he was depressed or had suicidal or homicidal thoughts. He said he didn't. Also in August, CNN has learned Alexis traveled to a Buddhist temple in Massachusetts and asked the monks if he could spend the night. They suggested a hotel, according to temple spokesman Eang Tan, but Alexis reportedly said he didn't like to stay at hotels because there were noises there that bothered him.

Disregarded warning: A senior Navy officer told CNN 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 MOTHER

"So very sorry": Alexis' mother apologized Wednesday to the families of the victims, saying she didn'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 said. "To the families of the victims, I am so, so very sorry that this has happened. My heart is broken."

THE INQUIRY

Thorough review: On Wednesday night, the Navy secretary ordered three reviews:

1) To see if Alexis' behavior on and off duty made him fit for duty or to hold security clearance.

2) To see if a contractor is required to inform the Navy if it reviews an employee's clearance.

3) A thorough look at how security clearances are granted and renewed in the Navy.

"Where there are gaps, we will close them. Where there are inadequacies, we will address them. And where there are failures, we will correct them," said Defense Secretary Hagel.