Thirteen people, including one gunman, were killed Monday morning in a shooting spree at the Washington Navy Yard.
Authorities said Monday night that they were still searching for a potential second suspect who may have disguised himself in military clothing, though they did not know for sure whether that person was involved in the attack.
Here's what we know about the attack so far:
Thirteen dead, a dozen others wounded
Thirteen people were killed in the attack, including the gunman. That made it the deadliest incident at a U.S. military facility since the 2009 Fort Hood, Texas, shooting.
Around a dozen others were wounded, according to the Washington Post, two of them police officers. One was shot twice in the leg while exchanging gunfire with the suspected shooter. Three people were left in critical condition, though officials said they are all expected to recover.
The shooter was a former Navy man
Law enforcement officials identified the suspected attacker as 34-year-old Aaron Alexis, a former Navy reservist. Born in Queens, N.Y., Alexis served in the Navy Reserve from 2007 to 2011.
It was not immediately clear how or when Alexis was killed, though police said he was found dead after one of multiple skirmishes with law enforcement.
The FBI has asked that anyone with additional information about Alexis contact the bureau at 1-800-CALL-FBI.
"No piece of information is too small," said Valerie Parlave, assistant director of the Washington field office of the FBI. "We are looking to learn everything we can about his recent movements, his contacts, and his associates."
The shooter had previous brushes with the law
Alexis was charged in 2010 with a shooting in Fort Worth, Texas, after a resident of his apartment complex said Alexis fired a gun through her floor, according to the Huffington Post. Alexis told police "that he was trying to clean his gun while cooking and that his hands were slippery," according to the police report.
In 2004, Alexis was arrested in Seattle, also on gun charges. He was accused of shooting out the tires of a construction vehicle, according to the Seattle Police Department. Alexis, construction workers said, "stared" at their work site every day for a month before the shooting.
From the Seattle Police Department:
Following his arrest, Alexis told detectives he perceived he had been "mocked" by construction workers the morning of the incident and said they had "disrespected him." Alexis also claimed he had an anger-fueled "blackout," and could not remember firing his gun at the victims' vehicle until an hour after the incident.
Alexis also told police he was present during "the tragic events of September 11, 2001," and described "how those events had disturbed him." [Seattle Police Department]
There may have been a second shooter
Officials on Monday night said they were still seeking information on a man believed to have been wearing a khaki uniform and carrying a pistol.
Initial reports offered conflicting information, with some claiming there were up to three gunmen roaming the compound. Police have since indicated Alexis was most likely a lone gunman, though they have not officially ruled out the possibility of a second shooter.
"We're continuing to see if there are in fact additional shooters, but we have nothing to indicate that yet," one anonymous official told the New York Times.
However, D.C's deputy mayor for public safety tweeted Monday afternoon that the second male was no longer a suspect.
The white male being sought in connection to the #NavyYardShooting has been identified and is NOT a suspect.
— Public Safety in DC (@SafeDC) September 16, 2013
No known motive
Law enforcement officials said they still do not know why Alexis chose to attack the Navy Yard. However, they do not believe the attack was coordinated with a terrorist group.
"We don't have any reason to think that at this stage," D.C. Mayor Vincent Gray said.
The shooting started as people arrived for work
Around 8:20 am Monday, gunfire erupted inside the Navy Yard, a mixed civilian and military site where some 16,000 people work in 2.2 million square feet of office space and government buildings. The shooting began inside Building 197, home to the Naval Sea Systems Command, which employs 3,000 people, some of them civilians.
Reports indicate that Alexis opened fire inside the office's cafeteria, spraying bullets, before moving on to other parts of the building. Navy Yard buildings were soon placed on lockdown as hundreds of emergency personnel rushed to the scene.
Alexis had clearance to enter the compound, according to The Washington Post, which is secured with a high wall and checkpoints that require identification.
Ray Mabus, the secretary of the Navy, told CNN that he believed Alexis was working as a subcontractor for the Navy, which allowed him to access the site. Hewlett-Packard later confirmed that Alexis worked for a company that it had subcontracted to work on equipment for the Navy Marine Corps Intranet.
There was no threat to the White House or Congress
The shooting took place about two miles from Congress and four miles from the White House. However, there was never a threat to either location.
The Senate recessed Monday afternoon in response to the attack. "We're thinking about today's tragic shootings at the Navy Yard, about the victims and about their families," Senate Minority Leader Mitch McConnell (R-Ky.) said.
There was a report of gunfire outside the White House later in the day, though that turned out to be an untimely prank; a man was arrested after throwing firecrackers over the White House fence.
Flags at the Capitol will be flown at half-mast for the rest of the week to mourn the shooting.
— SenateSergeantAtArms (@SenateSAA) September 16, 2013
Obama hailed the victims as "patriots"
President Obama prefaced a scheduled address on the anniversary of the 2008 economic collapse by saying he had been informed throughout the day about the attacks. He vowed to ensure that "whoever carried out this cowardly act is held responsible," and called the slain victims "patriots."
Baseball in D.C. was postponed
The Washington Nationals postponed a scheduled game against the Atlanta Braves. The two teams were slated to play just a few blocks down the road from the shootings, but will instead play a doubleheader Tuesday.
Both teams are in the playoff hunt.
Senator already calling for renewed gun control effort
Sen. Dianne Feinstein (D-Calif.) issued a statement Monday afternoon urging her colleagues to once again take up gun control legislation. Feinstein was one of the leading proponents of a ban on assault rifles that died in the Senate earlier this year.
This is one more event to add to the litany of massacres that occur when a deranged person or grievance killer is able to obtain multiple weapons — including a military-style assault rifle — and kill many people in a short amount of time. When will enough be enough? Congress must stop shirking its responsibility and resume a thoughtful debate on gun violence in this country. We must do more to stop this endless loss of life. [Feinsten.Senate.gov]