Aaron Alexis, a 34-year-old former Navy reservist, shot and killed 12 people Monday morning at the Navy Yard in Washington before police bullets ended his life:

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.

Ashley Halsey III, Aaron Davis, and Michael Laris

One Navy captain, Christopher Mercer, helped officers locate Alexis inside his building:

Mercer became the closest thing that hundreds of police officers descending on the Navy Yard on Monday had to eyes and ears on Alexis as the gunman moved through the third floor of Building 197, stalking the offices of some of the Navy’s most senior amphibious ship builders and buyers. . . .

Mercer, on the floor under his desk and armed only with his BlackBerry, began firing off e-mails to his commanders as he heard the door to the emergency staircase behind his office wall fly open.

The gunman, it seemed, was running down the stairs, toward the alleyway adjacent to the power plant where many of Mercer’s staffers had just fled. There were two shots. One of the two bullets, Mercer can only now imagine, was the shot that killed the maintenance worker standing beside Navy Cmdr. Tim Jirus in the alley.

Minutes went by, and then several more. Mercer, who leads a group designing a new $4 billion aircraft carrier to ferry the joint strike fighter, felt safe enough to climb out and sit at his desk. . . .

Mercer was also now in communication via e-mail with his boss, one of the leaders of the Senior Executive Service, who was standing outside Building 197 beside a police commander relaying his account.

“The gunman was here,” Mercer wrote. “Is he still there?” his boss asked. “I can’t tell,” Mercer wrote. “We’re still barricaded in my office.”

Stay there, they were told. Mercer nervously passed the time clicking through Microsoft Outlook, pulling up the office numbers for Arnold and others around him. He relayed the numbers to police, to tell them where the gunman had been.

Then, there was the sound of footsteps outside again. Mercer and his terrified colleagues slid back behind the desk. The could hear Alexis reloading his weapon.

Halfway down the west side of the building, where the looming smokestacks of the power plant next door can be seen through the windows, the gunman began to make his own last stand, sometime around 9 a.m.

“He was next door, through a little gypsum wall. He was moving furniture,” Mercer said. Mercer knew the office number. He e-mailed the coordinates: 3(W)20820.

Within minutes he could hear police. And then an eruption of gunfire.

“It was a fierce, major gun battle. Bullets were flying through my office, over our heads, and kept going for minutes,” Mercer said. “Then, I heard ‘shooter down, shooter down.’ ”

It was 9:25, Mercer remembered, when police pulled him and the three others from the office into the hall.

Mercer’s office was a wreck: Glass from frames on the wall was shattered; the floor was littered with the confetti of bullet-shredded wallboard and paper.

Aaron Davis

Alexis had a valid pass to several military installations, despite a history of misconduct, violence and mental illness:

The revelations about Alexis’s troubled past — and his ability to pass the government’s security-check system — prompted multiple examinations Tuesday into how background checks are conducted and how long a security clearance can last without review. The system was already under scrutiny after leaks of classified documents by fugitive National Security Agency contractor Edward Snowden.

President Obama directed his budget office to conduct a government-wide review of security standards for contractors and employees across federal agencies. Defense Secretary Chuck Hagel also ordered a broad review into security and access to military installations worldwide.

More than 24 hours after the deadly rampage, there was still widespread confusion over how Alexis managed to escape scrutiny since being given access to classified materials and facilities five years ago. The private contractor that most recently employed him pointed the finger at the Defense Department, which defended its handling of the case.

Alexis was granted secret-level security clearance in March 2008, when he was working as a full-time Navy reservist, according to the Pentagon. He was discharged from the Navy in January 2011 after a series of run-ins with his military superiors and police.

In September 2012, a Hewlett-Packard subcontractor called The Experts hired Alexis and said it confirmed his security clearance with the Defense Department. Thomas Hoshko, the company’s chief executive, said he confirmed the status again in late June of this year, when Alexis returned to work for the firm after a brief hiatus.

A background check done by a private contractor at the time turned up only a minor traffic violation, according to Hoshko. “It came back clean,” he said.

Carol Leonnig, Matea Gold, and Tom Hamburger

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