WASHINGTON, March 9 — Bipartisan outrage erupted on Friday on Capitol Hill as Robert S. Mueller III, the F.B.I. director, conceded that the bureau had improperly used the USA Patriot Act to obtain information about people and businesses.

Mr. Mueller embraced responsibility for the lapses, detailed in a report by the inspector general of the Justice Department, and promised to do everything he could to avoid repeating them. But his apologies failed to defuse the anger of lawmakers in both parties.

“How could this happen?” Mr. Mueller asked rhetorically in a briefing at the headquarters of the Federal Bureau of Investigation. “Who is to be held accountable? And the answer to that is I am to be held accountable.”

The report found many instances when national security letters, which allow the bureau to obtain records from telephone companies, Internet service providers, banks, credit companies and other businesses without a judge’s approval, were improperly, and sometimes illegally, used.