WASHINGTON - Much of the surveillance video taken during a March 4 incident at the White House complex, in which Secret Service agents drove into a barricade marking off a bomb-threat investigation, has been erased, the agency's top official told lawmakers Thursday.

Director Joseph Clancy, testifying before Congress for the second time this week, said the footage of the incident no longer exists because of an agency practice of recording over surveillance video every three days.

Clancy said his department was bringing in the security system's manufacturer and government experts to try to recover the lost footage. "We understand it's a concern," he said. "We're doing everything we can to retrieve those images and be as transparent as we can be."

Clancy's confirmation that his agency erased evidence that could be useful in an investigation of alleged staff misconduct on the White House grounds came as he tried to play down the seriousness of the incident.

That night, two senior agents, including a top member of President Obama's security detail, drove their car after a work party into an area cordoned off after a woman threw a package and yelled that it was a bomb. Surveillance video that had been preserved due to the bomb threat showed that the car drove next to the package, which was being investigated by police and later ruled to be harmless.

Clancy said Thursday that media reports had been exaggerated. He said that the recordings that had been preserved showed the agents' government car was moving very slowly and bumped a construction barrel out of the way.

Lawmakers reacted with frustration to Thursday's news.

House Oversight Committee Chairman Jason Chaffetz (R., Utah) said it was a "flat-out dumb policy" that would allow surveillance video that had been taken on the night of a bomb threat to be erased. He said the investigation would "drag some people before Congress and find out if it was done on purpose."

Secret Service spokesman Brian Leary said that, as a practice, the agency "maintains video footage of camera systems at the White House for a period of 72 hours." He said that, in the event of an "operational security incident at the White House complex, specific video footage is maintained for investigative and protective intelligence purposes."