WASHINGTON — Angry lawmakers on Friday threatened to amend the USA Patriot Act and limit the FBI's powers in the wake of a disclosure that agents had improperly obtained confidential records of people in the United States.

A scathing report issued Friday by the inspector general of the Justice Department found widespread problems in how the FBI has used a form of administrative subpoena — known as a national security letter — to gather phone, bank and credit information on thousands of citizens without court oversight.

The problems included the issuing of letters that circumvented Justice Department rules and regulations; in addition, the report found a record-keeping system in such disarray that annual reports to Congress substantially understated the number of subpoenas the FBI was issuing.

The inspector general also disclosed that the bureau had an unusual contract with three phone companies to provide call records and subscriber information without legal process.

The revelation was a major embarrassment for the FBI, which had vowed to use its investigative powers carefully when Congress reauthorized the Patriot Act last year.

National security letters do not require the approval of a judge, and have long been popular with law enforcement. The 2001 Patriot Act made them even easier to get in terrorism and espionage cases. The act also for the first time permitted FBI agents in the field to issue the letters; that authority had previously been reserved for officials at FBI headquarters.

FBI Director Robert S. Mueller III on Friday took responsibility for failing to establish an adequate monitoring system for the anti-terrorism measure. "How could this happen, who is accountable? And the answer to that is I am to be held accountable," Mueller said in a briefing with reporters. He cited problems with training and oversight of personnel, as well as the bureau audit system, and announced a number of steps to overhaul the process.

Mueller's boss, Atty. Gen. Alberto R. Gonzales, pointedly criticized the FBI and its director for falling down on the job.

"During the discussion of the reauthorization of the Patriot Act, I believed that the FBI was acting responsibly in using national security letters," Gonzales told a conference of privacy experts Friday. "Because of the good work of the IG, I've come to learn that I was wrong."

Gonzales said the Justice Department's Office of Professional Responsibility had opened an investigation into possible misconduct by lawyers at the FBI who failed to monitor the subpoenas.

"Once we get that information, we'll be in a better position to assess what kinds of steps should be taken," Gonzales said after his speech. "There is no excuse for the mistakes that have been made, and we are going to make things right as quickly as possible."

Two influential senators Friday expressed anger at the inspector general's disclosures and said they were considering tightening the Patriot Act regulations that allow the FBI to use the national security letters with such wide latitude.

Sens. Patrick J. Leahy (D-Vt.) and Arlen Specter (R-Pa.) also said they would call Mueller and Gonzales to testify in the coming weeks to get more answers and determine how widespread the problem is. Leahy is chairman of the Senate Judiciary Committee, which has oversight of the Justice Department and the FBI, and Specter is its ranking Republican and former chairman.

"The inspector general's report shows a massive misuse by the FBI of the national security letters for law enforcement," Specter said. "There'll be oversight hearings. And I think we may have to go further than that and change the law, to revise the Patriot Act … and perhaps take away some of the authority which we've already given to the FBI, since they appear not to be able to know how to use it."

The report by Inspector Gen. Glenn A. Fine presented a picture of mismanagement and self-regulation gone awry. Fine said he had no evidence of intentional wrongdoing, but found numerous examples of FBI personnel violating internal guidelines and procedures, as well as a failure to establish clear policies.

The report found that the FBI had greatly underreported the number of problems with national security letters to the President's Intelligence Oversight Board. And it indicated that the violations the FBI did report were less serious than ones that Fine and his investigators uncovered independently.

The FBI reported just 26 possible violations to the White House oversight board between 2003 and 2005, most of which were minor, such as "typographical errors," the report found.

But the watchdog report indicated that hundreds, or even thousands, of potentially more serious violations went unreported. Fine said a review of 77 FBI case files in four field offices found that 17 of the files, or 22%, contained violations that had not been identified by the field office or reported to FBI headquarters as required. Among the violations of policies and procedures:

•  A letter for telephone billing records was issued 22 days after the authorized period for the investigation had lapsed.

•  Full consumer credit reports were obtained during espionage investigations, even though the law says the information should only be available in international terrorism cases.

•  Educational records were improperly obtained from a North Carolina university.

•  Unauthorized information about phone numbers was received in 10 cases because of transcription errors and other problems by phone company providers.

Investigators also alleged that FBI headquarters circumvented the rules by obtaining billing records and subscriber information from three telephone companies on about 3,000 phone numbers without issuing national security letters at all.

The law allows the FBI to obtain such records under "exigent" circumstances. But the report found that the bureau, with the support of the phone companies, was using the power in non-emergency situations. The records were supplied between 2003 and 2005. The report found even top FBI lawyers were unaware of the practice until the latter part of 2004.

The report did not name the phone companies that received letters or cooperated with authorities by divulging call information. Sources said those companies probably were among the major long-distance carriers at the time — AT&T Corp., MCI Inc., Sprint Corp. or Qwest Communications International Inc.

"Generally, every day, Verizon units respond to emergency requests from federal, state and local authorities for calling records," said Peter Thonis, spokesman for Verizon Communications Inc. He would not comment, however, on whether the company or MCI — which it acquired early last year — was involved in the FBI effort.

AT&T spokesman Walt Sharp referred questions to the FBI.

Sprint, now called Sprint Nextel Corp., would not disclose whether it received one of the letters. "Generally, Sprint doesn't comment in situations where there is an ongoing investigation," spokeswoman Kathleen Dunleavy said.

Qwest, which has turned down other wide-ranging government requests for information in national security cases, also declined to comment.

The inspector general's report found that the number of national security letter requests grew from 8,800 in 2000 to a peak of 56,000 in 2004. The total issued in the three-year period covered by Fine's review was 143,074.

"The authority got decentralized, and what appears to have happened is that the FBI never built the proper processes for accountability and review at the field level," said Michael Woods, a former head of the FBI national-security law branch who once reviewed national security letter requests.

"When all the requests went through my office, we had a really good idea about the legal standards, but it was slow," Woods said. "People were screaming about the delays."

The inspector general's report resulted from a concession made to Democrats and other critics of the Patriot Act during the debate over renewing the law last year.

The Bush administration had fought new restrictions on the use of national security letters but acceded to the inspector general conducting periodic reviews for the public.