A man threw firecrackers over the North fence of the White House Monday afternoon, hours after a shooting at the Washington Navy Yard left more than a dozen dead.

The Associated Press said the middle-aged man, who was white with dark hair and wearing a blue shirt and white shorts, was arrested outside the gate on Pennsylvania Avenue. As agents responded, press and staff were ordered to retreat from the lawn into the White House complex, which was then locked down.

The initial sound of the firecrackers prompted concern from journalists in the area that someone had fired a weapon.

Earlier Monday, President Obama heralded the victims of the Washington Navy Yard tragedy as "patriots" and vowed to do "everything in our power" to make sure those responsible were brought to justice.