Flags over the White House and U.S. Capitol were ordered to half-staff late Monday as official Washington reeled in the aftermath of a deadly shooting at Washington, D.C.'s Navy Yard.
President Barack Obama ordered flags over the White House to half-staff through Friday evening to mark the tragedy in D.C., which claimed the lives of 13, including the gunman. House Speaker John Boehner ordered flags over the Capitol to half-staff for the same period of time.
The responses came after a tense day following the mass shootings just blocks south of the Capitol, which prompted a partial lockdown of the United States Senate complex out of “an abundance of caution.
As the city remained on edge, residents were rattled by reports of loud popping noises outside the White House, later attributed to a man who threw fireworks over a fence at the White House North Lawn. Secret Service has arrested the man; officials say he was not a threat.
Security personnel have now lifted restrictions on exiting Senate buildings but said that only staff members will be able to enter those facilities until Tuesday morning.
"Although we do not have any more information on the second alleged shooter, given the time of day and your personal family obligations, I am lifting the portion of the lock down which required you to remain in the buildings," Senate Sergeant at Arms Terrance Gainer said in a statement issued to Senate staff.
Gainer announced earlier Monday that individuals would be prohibited from leaving or entering Senate buildings until 5pm ET “in light of the uncertainty surrounding the shooting at the Navy Yard this morning and particularly the possibility of suspects remaining at large.”
The White House has also canceled a planned concert Monday evening celebrating Latin music and Hispanic Heritage Month.
The Obama administration said it would reschedule the performances -- featuring Gloria Estefan and Ricky Martin, among others -- for a later date. (The show is set to air on PBS on Oct. 8.)
The initial decision to lock down the Senate office buildings was to allow Capitol police to gather information and concentrate on individuals who may have been approaching the area, authorities said.
"While this approach is inconvenient, it is at times necessary. The lessons of Boston, Virginia Tech, Sandy Hook, and Aurora are clear, and just still too raw," Gainer wrote.
The Capitol Visitor Center was also temporarily closed Monday, but the United States House -- which is closer geographically to the site of the Navy Yard shooting than the Senate office buildings -- remained open.
The Senate, which was scheduled to vote on two judicial nominations on Monday evening, was recessed for the day and the votes postponed, Senate Majority Leader Harry Reid said on the Senate floor.
Reid also noted that he had seen Capitol police carrying heavy automatic weapons not typically carried by security forces on a day-to-day basis.
The shooting at the Sea Systems Command headquarters around 8:20 AM ET left at 13 dead, including the shooter, officials said. Law enforcement sources have identified Aaron Alexis as the gunman killed in a shootout with SWAT officers.
NBC's Michael O'Brien contributed