Updated 10:30 p.m.
Officials still don't know what motivated a 34-year-old man, Aaron Alexis, to open fire Monday morning at the Washington Navy Yard, killing 12 people and wounding a number of others.
Authorities initially said they were looking for two other suspects or persons of interest, but as the day wore on, D.C. police said they were certain Alexis acted alone.
“We do now feel comfortable that we have the single and sole person responsible for the loss of life inside the base today,” said Police Chief Cathy Lanier.
The victims range in age from 46-73, said DC Mayor Vincent Gray. Seven families have been notified of their deceased relatives, six are yet to be contacted. Eight people were injured in the attack—three of those were injured from gunshot wounds, including one law enforcement official. The other five have injuries ranging from extreme stress to various contusions; all eight are expected to make recoveries.
"The response of the police, the taking down of the shooter so quickly, convinces me yet again that this is the safest city in the United States--not safe from attack, but safe," Washington D.C. Rep. Eleanor Holmes Norton said at the press conference. Norton promised that "when I go on the floor tomorrow evening, I'll remind the Congress that this shooting occurred in their neighborhood."
The attack began on a drizzly Monday morning just before 8:30 a.m. as some of the Navy Yard’s 3,000 workers filed into their offices for the start of the work week.
Authorities believe Alexis arrived at the Navy Yard carrying only a shotgun, which investigators say he bought just last week at a gun dealer--Sharpshooters--in Lorton, Virginia, about 20 miles from Washington.
They believe that he then picked up two other weapons as he began the shooting spree, taking one of them from a police officer he shot.
When he was killed, authorities recovered three weapons--the shotgun, a handgun, and an AR-15-style assault rifle.
“It was three gunshots straight in a row: pow, pow, pow,” said Patricia Ward, who heard the gunfire from the Naval Sea Systems Command Headquarters. “Three seconds later, it was pow, pow, pow, so it was like about a total of seven gunshots and we just started running.”
Officials “don’t have any reason at this stage to suspect terrorism, but certainly it hasn’t been ruled out,” Mayor Gray said.
Commander Tim Jirus said that he heard what sounded like the pop of a gun around 8:30 in the morning. As part of the Safety Section tasked with safely evacuating employees in the building, Jirus got people to safety.
"There was an individual who came from the building behind us," Jirus said. "He came up behind us and was talking to me basically saying, 'hey, there's a shooter in your building,' and then I heard two more shots. One of them hit him, he went down in front of me and then I took off from there."
At least one first responder, a male officer from the Metropolitan Police Department, was injured and taken to Washington Medical Center in critical condition. Two women, one who was shot in the shoulder and another one with wounds to the head and hand, were also treated at the Center.
The woman who suffered a shoulder injury will be in surgery for a couple of hours, Dr. Janis Orlowski of the Center said during a press conference.
“She’s in very, very good spirits. She was actually ordering the doctors and nurses around and we told her that we were in charge here,” Orlowski said.
The third victim will not undergo surgery because the bullet “did not penetrate the skull, which means the bullet did not penetrate the bone,” Orlowski said. “I expect all three of them to make a full recovery.”
The FBI, Lanier said, will now lead the investigation.
Employees at the Naval Sea Systems Command Headquarters, where both civilians and military personnel design ships and submarines, have to pass through several layers of security before gaining entrance to the premises, Rick Mason, an employee at the facility told NBC’s Andrea Mitchell.
The shooter was employed as a subcontractor for Hewlett Packard to work on the Navy's intranet; that may have given him access to the base.
"As I was walking up the stairs I heard what I thought was a locker door slamming; it didn’t sound like a shot to me,” a witness, Brian Chaney, told MSNBC’s Craig Melvin.
The Senate sergeant-at-arms, Terry Gainer, said to NBC News that he recommended that Senate Leadership, including Majority Leader Harry Reid and Minority Leader Mitch McConnell, also shelter in place out of an "abundance of caution."
Capitol Hill is less than a mile away from the Navy Yard.
President Obama registered the shock of having a secure military facility come under attack during scheduled remarks at a press conference marking the anniversary of the financial crisis.
“They know the dangers of serving abroad but today they faced the unimaginable violence that they wouldn’t have expected to face here at home,” Obama said.
The president tied the shooting to other instances of gun violence.
“We are confronting yet another mass shooting,” Obama said. “And today it happened on a military installation in our nation’s capital.”
Obama had pushed for stricter gun control legislation following the mass murders in an Aurora movie theater and at Sandy Hook Elementary School in Connecticut. But a bipartisan bill requiring universal background checks and restricting the size of gun magazines died on the floor.
Monday night’s baseball game at Nationals Park was rescheduled to Tuesday at 1 p.m. EST.
NBC News' Pete Williams, Jim Miklaszewski, Courtney Kaube contributed reporting to this article. MSNBC's Michele Richinick also contributed to this report.