On Sept. 16, 2013, Aaron Alexis walked into Washington, D.C.'s Navy Yard complex and opened fire. He killed 12 people and wounded four before he was fatally shot by police. Here's a timeline of how the shooting rampage unfolded that day. (Gillian Brockell/The Washington Post)

In the end, after more than an hour of slaughter, it came down to a gunfight in a warren of cubicles in a very ordinary office building.

Aaron Alexis, emotionally unstable and armed with a shotgun and pistol, had moved from floor to floor in a high-security building at the Washington Navy Yard on a September morning, indiscriminately mowing down a dozen people.

Now police had him trapped amid the office spaces on the third floor.

The investigative report obtained from District police Friday spells out the death of the 34-year-old gunman.

As two D.C. police officers and a pair of agents from the Naval Criminal Investigative Service worked their way down a narrow path among the cubicles, Alexis rose up shooting. The lead District officer went down, struck in both legs, and the others dragged him to safety. Returning with reinforcements, they made their way to the last bank of cubicles when the gunman jumped up firing again.

“Alexis is approximately five feet from the officer when he fires,” the report says. A bullet struck the officer’s chest, but his protective vest saved him. The officers shot back.

“At 9:25, Alexis is hit by the return fire and is killed instantly, ending the threat 69 minutes after he fired the first shots,” the report says.

Although the 82-page investigation by the Metropolitan Police Department is rich with detail, it does not name any of the victims or their co-workers. It draws no conclusion about why Alexis went to the Navy Yard — where he had worked as a contractor for only a week and into a building he had entered just a few times — and began shooting.

“While it is human nature to want to know the specific reasons why Alexis may have carried out this tragedy,” the report says, “his motive is beyond the scope of this review.”

Here are excerpts from the report, augmented by Post interviews with witnesses after the shooting and in recent weeks. Together, they provide the most complete story to date of the crime.

MPD report, 8:08 a.m.: “Alexis enters Building 197 through the main entrance on the east side of the building, scans his access identification card. . . . Alexis takes the elevator to the 4th floor, where he conducted work during the prior week, exits the elevator and casually walks directly to the men’s restroom.”

Stepping into a restroom stall, he pulls a Remington 870 shotgun and a supply of shotgun shells from his backpack and spends several minutes getting ready for what is to come.

MPD report, 8:16-8:20: “The first 911 caller works on the fourth floor. . . . When he hears the first shots, he believes the sounds resemble the noise of a large safe being dropped.”

“They’ve dropped safes before, and it’s awfully similar to a shotgun going off,” said John Weaver, who placed that 911 call, in an interview with The Washington Post after the shooting. “The first shot I thought was a safe falling, but when the second one went off, I knew there was a person in the building shooting people.

“The first bang immediately after, that’s him shooting Frank [Kohler], and a second bang that’s him shooting J.J. [John Johnson],” Weaver said after the attack. “That quick: bang-bang.”

MPD report, 8:16-8:20:He stands up and looks over the cubicle partition and observes Alexis point the shotgun and fire at a young woman sitting at a cubicle.”

“Her scalp and hair are all over the cubicle,” Weaver said. “But she did not get a penetration wound. She survived.”

Two of those who were shot and survived said later that Alexis locked eyes with them before pulling the trigger.

“The next person he shot was Mary Knight,” Weaver said.

“After he shot Mary, I went down onto the ground and hid under my desk,” Weaver said. “He didn’t pursue people under desks. He basically shot at anybody he saw moving. If he saw you, he was coming after you.”

MPD report, 8:16-8:20:. . . the first caller is also able to provide a detailed description of the shooter.”

He wasn’t wild-eyed, Weaver recalled. “His demeanor was pretty damned calm for somebody who was going around killing people.”

The District’s 911 call center would receive 62 calls between 8:17 and 11 a.m.

“He saw [Sylvia Fraiser and Gerald Read] after he dropped Mary,” Weaver said. “He chased them as a group from the west side of the building to the east side of the building, straight down that corridor. He shot Sylvia first, then Gerry. Gerry pushed a partition wall between them, and he shot Gerry through the partition wall.”

MPD report, 8:16-8:20: “Within the first four minutes of firing his first shots, Alexis shot and killed eight victims on the fourth floor. He then makes his way to the third floor.”

Weaver didn’t know where he’d gone. “ I sprinted out my cubicle,” Weaver said. “I was crouched down and running.”

Then he came upon Mary Knight’s body. “I looked her right in the eye. There was just no life in her eyes, and I was like, ‘I got to move.’

“I ran down the main stairs, and what was really amazing was how many people were just milling about as if there was nothing going on,” Weaver said. “I went through yelling at all these people to get out of the building, that there was a man with a shotgun, and he was murdering people on the fourth floor.”

MPD report, 8:20-8:23: “Within the first two minutes on the third floor, Alexis fatally shoots two more victims. . . . He sees a young woman hiding between a metal beam and filing cabinet. He stands directly over her and attempts to fire twice, but the shotgun does not fire. As he enters one of the stairwells on the third floor, he encounters several people coming down the stairs from the fourth floor. Before they can all safely make their way back up the stairs and away from the shooter, Alexis fires once, striking a woman in the shoulder.”

It was Jennifer Bennett, 57. “I could see he was angry,” Bennett remembered last month. “Then I could see him making a decision. I watched him pull the trigger. I watched the explosion hit me.”

Navy Capt. Edward “Chip” Zawislak heard her call and rushed with others to help her. They carried her to the roof, dropped a note to an officer on the ground, asking for a helicopter to pick her up and applied pressure to her wounds as they waited.

As the sound of gunfire continued in the 600,000-square-foot building, some men used their neckties to help secure doors behind which they were hiding.

Out of the building, Weaver said he rushed toward the infirmary with the woman whose hair had been shaved off by the shotgun blast.

“On the way, I encountered probably about 100 police officers running toward the building,” he said. “There was a Navy Yard officer that I recognized, and I told him that there was a guy up on the fourth floor with a shotgun.”

MPD report, 8:27-8:34: “[Alexis] eventually makes his way through the south side of the first floor and around to the front entrance of the building where he encounters the security guard.”

Alexis had passed Mike Ridgell’s station just minutes before when he entered the building.

MPD report, 8:34-8:40: “Before the guard can react, Alexis fires at him, hitting the guard and shattering the glass windows of the front entrance. Alexis then retrieves the sidearm of the mortally wounded guard and proceeds north down the hallway.”

Now Navy Yard police and security people were in the building. They exchanged shots with Alexis twice, but both times he disappeared. At the end of a hallway he saw two men standing in the alleyway. He opened the exit door and raised his shotgun, but apparently it was out of ammunition. Instead, he used Ridgell’s pistol to shoot one of the men dead, the last of his 12 victims. He dropped the shotgun and moved on.

With teams of police moving into the building, it would take another 47 minutes for them to confront him.

MPD report, 8:40-8:55: “His demeanor has visibly changed. He has gone from hunter to the hunted.”