Read eyewitness accounts from the Navy Yard shooting.

Aaron Alexis arrived at the Navy Yard with his backpack slung over his shoulder, drawing no particular attention as he flashed his ID card and joined the stream of workers passing by the armed guards posted at the brick-and-wrought-iron gates.

He entered Building 197, tucked in one corner of the complex and overlooking the Anacostia River, on a cloudy and unseasonably chilly Monday morning without attracting undue notice, making his way toward his cubicle.

He slipped into a fourth-floor men’s room and then — about 8:15 a.m. — everyone’s day changed.

As investigators piece together the critical details of a mass shooting that traumatized a military facility just south of Capitol Hill, bits of preliminary information — from law enforcement officials and others — provide pieces of the puzzle.

From the backpack, Alexis pulled a Remington 870 shotgun. He purchased it Saturday, along with a couple dozen shells, from a gun shop in Lorton, where he took some time to fire a few practice rounds on the shop’s range. He emerged from the men’s room Monday prepared to use it.

In the wake of Monday's shooting at the Washington Navy Yard, The Post's Pentagon Correspondent Ernesto Londoño speaks with us about how the military will need to reassess its security protocols. (The Washington Post)

8:23 a.m.: D.C. police received the first report of gunshots from 1333 Isaac Hull Ave. — the address of the Naval Sea Systems Command, known on the base as Building 197.

Who got shot in the next 30-plus minutes was indiscriminate, just a matter of happenstance.

Capt. Mark Vandroff — a 1989 U.S. Naval Academy graduate — is a stickler for starting meetings on time. So Monday morning, the routine staff meeting began promptly at 8 a.m. in a third-floor conference room. It felt as if he had just begun handing out the week’s assignments and receiving reports.

“I heard the gunshots,” he said. “Someone screamed.” Someone yelled that there was a shooter. “Lock the doors! Lock the doors!”

The building has three main north-south hallways. Each opens onto an atrium. Sound carries from one floor to the next in ways it might not if it weren’t for the acoustics of the atrium. Then someone triggered the fire alarm.

Dozens of workers began to scramble.

“People were fleeing into offices,” Vandroff said. A few came into the conference room. They shut themselves in to “try to get another layer of protection” and barricaded the door with tables and chairs.

8:34 a.m.: An e-mail to Navy Yard personnel: “ALL HANDS on WNY. Shelter in place.”

Police teams that deal with ongoing shootings — four officers each, armed with AR-15 rifles — already were arriving at Building 197. Before it ended, there would be as many as seven teams involved in a fierce firefight.

The shots seemed to be coming from just south of where Vandroff and his colleagues were hiding.

By now, the police teams — joined by at least three naval security officers and U.S. Park Police — were moving in military fashion, stalking Alexis even as he stalked his victims on the fourth floor above Vandroff.

But the gunman had the advantage, familiar with the building’s layout and using the balcony wall for concealment as he fired the shotgun from the high ground into the atrium.

“He had the advantage, and no one knew where he was,” an official said. “He was moving. It was fish in a barrel.”

Gregory Dade heard a “pop-pop” from his second-floor office.

“We heard some rapid fire,” said Dade, who works for Hew­lett-Packard and is a contractor at the Navy Yard.

Opening their door, he and a colleague bolted, only to hear another burst of fire just ahead of them.

“We could smell the sulfur type of smell. You could just see it and smell it,” he said. They ran back to their office and locked the door. “What would you do, continue down the hall not knowing?”

8:35 a.m.: D.C. fire and rescue, emergency radio: “We got a report on the fourth floor. A male with a shotgun, multiple shots fired, multiple people. We’re still waiting for the okay that the scene has been secured.”

A colleague sent Vandroff a BlackBerry note telling him he was hiding with friends in a closet-size office cubicle, safe for the moment. But he heard nothing from the office of his friend Mike Arnold. Others also were out of contact.

“We didn’t know if something bad — or not bad — happened to them,” said Vandroff, whose group oversees the construction of Arleigh Burke-class destroyers.

Just down the hall, Capt. Christopher Mercer could hear the screaming, and the shots getting closer as Alexis made his way to the third floor.

Alexis turned left instead of right, and that made all the difference. He paused at the threshold of Arnold’s office door. Arnold was one of the Navy’s preeminent shipbuilders, but he was enamored with planes, and an aviation calendar was spread out on the desk in front of him when Alexis entered.

Alexis said nothing and pulled the trigger, sending a shotgun blast into Arnold’s chest.

The split-second delay gave Mercer and the three staffers across the hallway time to slam the door shut and begin pulling furniture in front. As soon as they backed away from the door, a bullet splintered the wood at shoulder height.

Mercer and his three staffers all dove under his desk.

“He set up camp right in front of my office,” Mercer said. “He kept reloading and firing at cubicles. Later, when he came back, I could see his shadow through the glass pane in my door.”

Huddled under his desk, Mercer heard Alexis pacing through the labyrinth of cubicles. He stumbled upon one of Mercer’s aides, a woman in her 20s, squatting for cover by a filing cabinet. Holding the shotgun to her face, he pulled the trigger, but nothing happened.

Alexis was making his way down the hallway on the fourth floor, shooting as he went. Then, for reasons unclear, he descended to the building’s lobby.

8:43 a.m.: D.C. emergency radio: “Have confirmation of at least five people shot. They’re attempting to bring outside, so right now police confirmation five people shot. Could be others.”

The lobby gunfight was brief and bloody. Confronted by a guard, Alexis shot him dead, scooping up the officer’s 9mm handgun and abandoning his shotgun.

With the response teams flooding into the building — there would be seven different points at which they exchanged fire with Alexis over at least 30 minutes — he retreated back upstairs, stopping this time on the third floor, where Vandroff and his friends were barricaded.

A bullet pierced the conference room’s wall.

“There was one shot that was very close. There were bullet holes in the wall of the conference room where we were hiding,” said Vandroff, who lives in Bethesda.

8:44 a.m.: D.C. emergency radio: “MPD is advising that they’re unable to hear each other due to the fire alarms. . . . The shooter is still on the loose.”

For Alexis, it ended when he popped from behind a partition in a third-floor office, the stolen pistol in his hand, to be greeted by a deafening explosion of gunfire. Stuck several times in the head, he went down.

And there might have been silence once he fell dead, but for the drumbeat of military and medevac helicopters swarming above the Washington Navy Yard, the clatter of the building's fire alarm, the shrieking sirens from scores of squad cars and ambulances, and the guttural barked commands of armed men moving to secure the building.

By now the people who had seen glimpses of the mayhem had raised the possibility there were several shooters — with at least two identified as wearing military-type uniforms.

9:12 a.m.: D.C. emergency radio: “Medical group on the details, please be advised. Scene is not safe. We are denied entry. Scene is not safe.”

By now, even as they continued to comb the building and fan out through the Navy Yard and into the surrounding neighborhood, police had recovered the shotgun Alexis used.

After a time, police sweeping through Building 197 reached Vandroff’s conference room.

“They announced themselves. You heard ‘bang, bang, bang’ on the conference room door,” he said. “ ‘Police!’ ”

The officers knew they might find a gunman behind any closed door.

“Open the door! Show us your hands! Open the door! Show us your hands!”

They put everyone from the room in a line, with an officer in front and an officer in back, and led them out of the building toward the north, avoiding the carnage.

Mercer wasn’t as lucky. He looked back, and it seemed his office was destroyed.

Glass in frames on the wall was shattered. The floor was littered with shreds of the wallboard and paper.

Outside, there were two officers with flak jackets and rifles standing at the entrance to the office where they had heard the gunman.

Hundreds of shell casings covered the floor.

“You could barely walk on the floor,” Mercer said. There was another officer standing guard over a weapon.

But that wasn’t the worst. A few steps to the south, as they walked out of a passageway into the main north-south hallway, “we literally almost had to step over” a body, Mercer said.

10:13 a.m.: Police announced that three shooters were involved — one was dead, and the search was on for two others.

The police instructed the workers to run across Isaac Hull Avenue and into the basement of a parking garage — protection of a sort if more shots were fired.

“We now had cars and concrete around us,” Vandroff said.

Military medical workers in the basement consoled the terrified workers. Some needed to go to the bathroom, and police led them on relief runs. A worker recovering from knee surgery, injured in all the hustle to rush from the building, got ice. A woman nine months pregnant was rushed by ambulance to a local hospital as a precaution.

“We were all clearly stressed out,” Vandroff said.

Eventually, police would determine that Alexis acted alone.

Colleagues would approach Vandroff about his friend Arnold, whose office is just down from his.

“Mike got hit,” one told him.

He was among the 12 dead.

Sari Horwitz, Peter Hermann, Tom Jackman, Ann Marimow and Clarence Williams contributed to this report.