"We cannot do our job without the public's help," said Cox, adding that the FBI had received more than 7,000 leads and tips so far.
The surveillance video showed a person wearing a hat, sunglasses and a long sleeve black shirt running across a roof, climbing off the edge of the roof and dropping to the ground. The suspect is believed to have fled into a neighborhood after firing one shot and has not yet been identified, officials said Thursday.
The circumstances of the shooting drew renewed attention to an escalating threat of political violence in the United States that, in the last several years, has cut across the ideological spectrum. The assassination drew bipartisan condemnation from political leaders.