The US Secret Service locked down the White House Saturday after a loud noise was heard as reporters gathered to await President Barack Obama and wife Michelle's departure for Selma, Alabama, a pool report said.
The source of the noise was unclear.
A Secret Service K9 unit was investigating a vehicle on 16th and Constitution Avenue which borders the South Lawn of the White House, agency spokesman Brian Leary said in an email.
The Washington DC Fire Department, meanwhile, reported a fire at a food cart on 15th Street NW on the east side of the White House. It said the fire was put out.
Security personnel shut down access to the White House, roping off Lafayette Park on the White House's northern side.
"As pool and other press gathered outside near the South Lawn for the departure of President Obama and the First Family on the way to Selma, Ala., for a historic 50th anniversary march, a loud noise was heard," a pool report said.
"The Secret Service then moved press back into the briefing room and locked the doors," it added.
Subsequent pool reports said the presidential motorcade left the White House and at 11:36 am Obama, Michelle, their daughters Malia and Sasha and Michelle's mother, Marian Robinson, departed for Selma aboard Air Force One.