WASHINGTON, March 9 — The F.B.I. has improperly used provisions of the USA Patriot Act to obtain thousands of telephone, business and financial records without prior judicial approval, the Justice Department’s inspector general said today in a report that embarrassed the F.B.I. and ignited outrage on Capitol Hill.

The report found that the bureau lacked sufficient controls to make sure that its agents were acting properly when they obtained records using administrative subpoenas, which do not require a judge’s prior approval. And the report found that the bureau does not follow some of the rules it does have on the matter.

Robert S. Mueller III, the director of the Federal Bureau of Investigation, called a news conference today to accept responsibility for the lapses, and to pledge his best efforts to see that they are not repeated.

“How could this happen?” Mr. Mueller asked rhetorically. “Who is to be held accountable? And the answer to that is, I am to be held accountable.”