After the gunman was also killed in a shootout with police, authorities warned there could be two other potential perpetrators still at large. But at a late-night news conference, Cathy Lanier, D.C.'s police chief, said Aaron Alexis, a 34-year-old one-time Navy reservist, acted alone.
The FBI appealed to the public to come forward with information about Alexis, who was working as a military contractor after being discharged from the Navy two years ago. He was born in New York City but most recently resided in Fort Worth, Texas.
In one of the most powerful cities on Earth, where security is of paramount concern, the notion of multiple perpetrators targeting a military institution raised the spectre of an orchestrated attack 12 years after al-Qaida terrorists commandeered a hijacked jetliner into the Pentagon.
The city's mayor, Vincent Gray, said earlier Monday that officials had no reason to believe the shooting was an act of terrorism.
"We'll continue to seek information about what the motive is," he said. "We don't have any reason at this stage to suspect terrorism, but certainly it has not been ruled out."
Officials are now investigating whether Alexis's discharge from the Navy played a role in the events. The Navy Yard is the largest of the U.S. Navy's five system commands.
In remarks from the White House, a terse U.S. President Barack Obama pointed out the regularity with which such shootings afflict the nation. Obama tried unsuccessfully to push tougher gun control laws through Congress earlier this year following the horrific shooting in Newtown, Connecticut in December.
"We are confronting yet another mass shooting, and today it happened at a military installation in our nation's capital," Obama said. "We will do everything in our power to make sure whoever carried out this cowardly act is held responsible."
The FBI said that despite Initial reports that Alexis entered headquarters at the Navy Yard complex with a stolen ID, he had a valid pass due to his work as a military contractor. He then fatally shot a security guard and eyewitnesses said he opened fire from above a cafeteria filled with people at about 8:15 a.m. EST.
The gunman situated himself at a fourth-floor overlook and was shooting down at people in the cafeteria on the first floor, creating panic and terror as people tried to flee the area. He had an AR-15 assault rifle, a shotgun and a pistol, police said.
"I heard three shots — pow, pow, pow. Thirty seconds later I heard four more shots," said Patricia Ward, a logistics management specialist who was in the cafeteria at the time.
"A lot of people were just panicking. There were no screams or anything because we were in shock."
It emerged that Alexis was the subject of a police investigation in Fort Worth three years ago when he was arrested for shooting a hole through the ceiling of his home and into a neighbour's apartment.
Alexis told police he was cleaning his gun when it went off accidentally, and there was no indication he was ever charged.
But in the police report, the neighbour said she suspected Alexis had deliberately fired through her floor because he had complained about her making too much noise. She added she was "terrified" of Alexis.
He was also investigated for shooting out the tires of a man's car in Seattle in 2004. His father told police his son suffered from post-traumatic stress disorder and had helped in rescue efforts in New York City following the terrorist attacks of Sept. 11, 2001.
Monday's shootings prompted the temporary lockdown of D.C.'s Ronald Reagan National Airport and schools surrounding the Navy Yard, a historic complex in the city's southeast that dates back to the 18th century and is now the workplace for about 3,000 people.
Throughout the day, people residing near the Navy Yard were ordered to remain in their homes. Those who worked on the naval base also had to stay indoors.
A major-league baseball game between the Washington Nationals and the Atlanta Braves was also postponed. The baseball stadium is just a couple of blocks from the scene of the shootings.
Well into the evening, as Navy Yard employees began filing out of the building, the city was still on edge. Reports of shots being fired at the White House turned out to be a case of someone tossing firecrackers over the presidential residence's iron fence, the Secret Service said. Police quickly apprehended the man.
The shooting made headlines on news sites around the world. A Russian official even weighed in — when the confirmed death toll was just seven — to mock Obama's insistence in a speech last week on Syria that the U.S. was "exceptional."
"A new shootout at Navy headquarters in Washington — a lone gunman and 7 corpses. Nobody's even surprised anymore," tweeted Alexey Pushkov, a member of the Russian parliamentary foreign affairs committee.
"A clear confirmation of American exceptionalism."
Sen. Dianne Feinstein, one of the U.S. Senate's biggest gun control advocates, called once again for stricter gun laws in the aftermath of yet another mass shooting.
"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," the California Democrat said in a statement.
"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."