[5][8][9][10] As shown on surveillance footage, he entered Building 197 at 8:08 a.m. through the main entrance, carrying a disassembled shotgun (its barrel and stock had been sawed off)[11] in a shoulder bag.

A NAVSEA employee described encountering a gunman wearing all-blue clothing in a third-floor hallway and said that "he just turned and started firing.

At the same time, a team of officers entered Building 197, but they became confused after gunshots echoed through the atrium, leading them to believe that he was on an upper floor.

He and 11 of the victims were killed at the scene,[20] while Vishnu Pandit, a program manager in the US Navy, later died at George Washington University Hospital.

Police officer Scott Williams and two female civilians were wounded by gunfire and were in critical condition at Washington Hospital Center.

[26] He joined the Navy in May 2007[10] and served in Fleet Logistics Support Squadron 46 at Naval Air Station Joint Reserve Base Fort Worth.

[30][31] He was also arrested in 2004 in Seattle, Washington, for malicious mischief after shooting out the tires of another man's vehicle in what he described as an anger-fueled "blackout",[25][32][33] and for disorderly conduct in 2008 in DeKalb County, Georgia.

Following the Navy Yard shooting, it was found that the federal personnel report, which led to the clearance's approval, did not mention that his 2004 arrest had involved a firearm.

[38][39] Alexis worked in Japan from September 2012 to January 2013 on the Navy Marine Corps Intranet network for an HP Enterprise Services sub-contracting company called The Experts.

A message later obtained by federal authorities from Alexis's personal computing devices said, "Ultra low frequency attack is what I've been subject to for the last 3 months.

[50] Five days later, he sought treatment for insomnia in the emergency room of a VA medical center in Washington, D.C., where he told doctors he was not depressed and was not thinking of harming others.

[51] At the time of the shooting, Alexis had been working for a subcontractor on a Hewlett-Packard Enterprise Services contract supporting a Navy-Marine Corps (NMCI) computer network.

At the time of the massacre, he had been staying with five other civilian contractors at the Residence Inn hotel he booked into in southwest Washington, D.C., since September 7.

[16][52] On Saturday, September 14, two days before the massacre, Alexis visited the Sharpshooters Small Arms Range in Lorton, Virginia, 15 miles (24 km) south of Washington.

After testing the rifle, Alexis inquired about buying a handgun at the store, but was told federal law does not allow dealers to sell such guns directly to out-of-state customers.

Alexis instead purchased a Remington Model 870 Express Tactical 12-gauge shotgun and two boxes of shells, after passing a state and federal background check.

[55] On the day of the shooting, Washington Chief of Police Cathy L. Lanier initially said that police were searching for a white male wearing khaki military fatigues and a beret, who had allegedly been seen with a handgun, and a black male wearing olive military fatigues and carrying a long gun.

The renovations, costing approximately $44 million, include a reflection area and new visitors' entrance, new flooring, furnishings, and an updated cafeteria.

[64] Shortly after news of the shooting broke, United States President Barack Obama pledged to ensure the perpetrators would be held responsible.

[65] Obama ordered flags at the White House, all public buildings and all military and naval posts, stations and vessels to be flown at half-staff until sunset on September 20.

[70] On September 18, Defense Secretary Chuck Hagel ordered a review of security procedures at military facilities around the world.

), provided to government contractors, civilian Defense Department employees, and soldiers, can enter many military facilities "without being patted down or made to go through a metal detector".

[73] Conservative commentators including Alex Jones, Ted Nugent, and others suggested that "gun-free zones" on military bases were to blame for the massacre.

[74] On NBC's Meet the Press, National Rifle Association of America leader Wayne LaPierre said, "when the good guys with guns got there, it stopped.

The activists said they hoped that the Navy Yard attack's proximity to Capitol Hill would motivate lawmakers to act to impose stricter background checks and close the gun show loophole.