A shooter opened fire at the Washington Navy Yard on Monday morning, killing 12 and injuring five others before getting shot by police. The shooting began around 8:20 a.m. with shots fired at Building 197, which contained approximately 3,000 employees.

The Navy Yard will be closed Tuesday, except for emergency response and mission-essential personnel.

Hundreds of Georgetown seniors, as part of the Senior Class Committee’s Dis-Orientation, were scheduled to attend Monday evening’s Washington Nationals game, which was postponed to 1 p.m. Tuesday in the wake of the shooting. Many students would have arrived via the Navy Yard Metro stop, which is less than a mile from the ballpark.

Monday’s shooting was the deadliest event in the District’s history since 1982, when an airliner plunged into the Potomac River, resulting in 78 deaths.

Mayor Vincent Gray said there were no indications that the shooting had a terrorist connection.

“We don’t have any known motive at this juncture,” Gray said in a press conference around 6 p.m.

Two of the injured were members of the Metropolitan Police Department, and both are in stable condition.

Among the dead was Aaron Alexis, the 34-year-old suspected shooter from Texas who carried an assault rifle, a handgun and possibly a shotgun.

In a release, the Navy confirmed that Alexis, who served in the Navy as a full-time reservist from 2007 to 2011, was a petty officer third class. The reasons for his general discharge are still unclear, though the Seattle Police Department reported that Alexis had previously suffered from anger-fueled blackouts, caused by post-traumatic stress disorder from participating in rescue attempts after the Sept. 11 attacks.

According to a police report, the Seattle police arrested Alexis in 2004 for shooting out the tires of a man’s vehicle during one of these blackouts. For an hour after the incident, Alexis claimed no knowledge of the shooting.

The Navy Yard, the country’s oldest military installation, is home to several high-ranking Navy personnel. Chief of Naval Operations Jonathan Greenert, a member of the joint chiefs of staff, was evacuated from his Navy Yard residence shortly after the first report of shots.

Navy officials said that the Navy Yard’s security is high, with gates manned by U.S. Marines and Naval District Washington security personnel. Visitors without military identification must present other identification as well as a valid reason for entry.

Alexis, who worked as a computer contractor for military contractor The Experts, had a government-contractor access card that allowed him entrance into the Navy Yard and other military installations. The Experts Chief Executive Thomas Hoshko said that Alexis had to pass a thorough investigation, conducted by military security service personnel, in order to receive his security clearance, which was only updated in July.

But hours after the shooting, authorities were still unclear about whether the shooting had been perpetrated by Alexis alone or by multiple shooters.

Initially, Metropolitan Police Department Chief of Police Cathy Lanier announced that, based on witness accounts and surveillance video, authorities were searching for two other suspects, but shortly afterwards, city officials announced that one of the suspects had been located and cleared of involvement. By Monday evening, however, Lanier announced that investigators were confident that Alexis was the only gunman.

In response to the shooting, the 11th Street Bridge, which leads to the Navy Yard, was closed. In addition, the D.C. Department of Transportation, the Van Ness building — which contains the D.C. Public Schools administration — and Eagle Academy Public Charter School were locked down due to their close proximity to the shooting. Flights out of Reagan National Airport were also briefly halted, and Nationals Park Parking Lot B was used as a safe zone for families to reunite in the immediate wake of the shooting.

As a precaution, Senate buildings were also locked down. In a message to sent Senate staffers at 3 p.m., Senate Sergeant at Arms Terrance Gainer wrote that staffers were not allowed to leave or enter the building for two hours or until the situation was deemed safe in the neighboring community, but the lockdown was lifted after one hour. Only Senate staffers will be allowed in the buildings until Tuesday.

In a press conference Monday afternoon, President Barack Obama stressed that the government would take severe action against those responsible for the shooting and expressed disappointment in the occurrence of the national tragedy.

“We’re confronting yet another mass shooting, and today it happened on a military installation in our nation’s capital. … These are men and women who were going to work, doing their job, protecting all of us. They’re patriots,” Obama said. “They know the dangers of serving abroad, but today, they faced the unimaginable violence that they wouldn’t have expected here at home.”

Obama ordered all flags in the White House and in all federal government and U.S. military facilities around the world to be flown at half-staff until sunset on Friday.

Students who were set to attend Monday night’s Nationals game can exchange their tickets for tomorrow afternoon’s make-up game, any other game this season — which ends Sunday — or any game in the 2014 season.

Given Georgetown’s location — about five and a half miles from the site of the shooting — Georgetown University Chief of Police Jay Gruber said that the campus alert level was not significantly affected. Nevertheless, all field officers, university officials and Department of Safety staff were notified in order to closely monitor the situation, and Gruber said he had several local and federal resources on hand.

“We want to make sure that they’re aware of any new information that comes out as a result of the investigation,” Gruber said in the afternoon. “Right now, there’s some suspect information, and we’ve put it out to our officers in the field.”

DPS also released a public safety alert to the campus community around 4 p.m., warning them to stay away from the Navy Yard neighborhood.

Fr. Pat Rogers, S.J., remembered the victims Monday at the regularly scheduled 10 p.m. Mass, and university chaplains will meet today to discuss any further response, according to Vice President for Mission and Ministry Kevin O’Brien, S.J.

Del. Eleanor Holmes Norton (D-D.C.), a professor at Georgetown University Law Center, tried to reassure D.C. residents during Gray’s Monday afternoon press conference.

“This is the safest city in the United States,” Norton said. “Not safe from attack, but safe.”