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 is 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 Barack Obama's security detail, drove their car following a work party at a Chinatown bar into an area that had been cordoned off after a woman had thrown 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 afternoon'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 committee's bipartisan investigation will "drag some people before Congress and find out if it was done on purpose."

Chaffetz and several lawmakers investigating the latest case of alleged Secret Service misconduct had expressed concern late Tuesday after viewing two videos of the incident during a closed meeting. They complained to Clancy that the footage did not help them see much of the agents' actions that night. The videos showed the same scene from two angles, according to lawmakers who viewed them.

When they asked Tuesday for more video of the incident and of the events leading up to it, Clancy was noncommittal. He went on to acknowledge the service's practice of recording over surveillance video, according to three lawmakers who were present.

Rep. Stephen Lynch, D-Mass., a ranking member of an Oversight subcommittee, said Thursday there was little excuse for such information to be purged so quickly, especially given that many businesses retain such video footage for a month.

"It is an antiquated system to purge video after 72 hours. We know from past experience that post-incident review relies heavily on our ability to retain those tapes," he said. "There is no reason to be taping over those tapes. We need a stem to stern review of our technology at the White House."

Rep. Elijah Cummings, D-Md., said he thinks the agency policy should be to save video on the night of a bomb threat.

"It's extremely concerning to me," he said. "Even without this incident, this is film we should have kept, film I would have liked to see. "

The March 4 incident, which is under investigation by the Department of Homeland Security's inspector general, has prompted renewed questions from lawmakers about the ability of the Secret Service's leadership to turn around their agency after a string of missteps. Clancy, who took over temporarily last fall after the resignation of then-Director Julia Pierson, was appointed last month to be the agency's permanent director.

On the night of the most recent incident, officers on duty believed the two senior agents were behaving erratically, according to officials familiar with the incident. Officers complained that a supervisor on duty that night ordered them to let the agents go home without facing sobriety tests, an official said.

While the two videos viewed Tuesday by lawmakers showed the agents' car driving near a suspicious package, several lawmakers told The Washington Post that the footage was not useful in weighing whether the agents had been driving under the influence of alcohol or behaved oddly with the officers.

Clancy's remarks Thursday shed light on the Secret Service's practice in preserving certain video footage but erasing routine recordings.

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."

Surveillance videos are normally saved onto a separate disk if there is a reported incident that day, in part to preserve evidence and to help with deeper investigation.

That typically includes video from the complex taken during any such incident, and for roughly an hour before and an hour after. That is to determine whether suspects can be seen somewhere else near the property, what they were doing and whom they were with.

On the night of March 4, the investigation into the suspicious package was considered a serious incident, and that explains why the Secret Service retained some video that shows the agents' path near the package. The senior agents' actions when driving onto the White House grounds were not reported internally as a serious incident.

Clancy said he did not learn of the incident until March 9-five days later, and two days after much of the video footage from that night had been erased. He said he learned of it after a whistleblower made an anonymous complaint. Clancy said he was "very frustrated" that he had not been immediately notified.