The FBI has conducted clandestine surveillance on some U.S. residents for as long as 18 months at a time without proper paperwork or oversight, according to previously classified documents to be released today.

Records turned over as part of a Freedom of Information Act lawsuit also indicate that the FBI has investigated hundreds of potential violations related to its use of secret surveillance operations, which have been stepped up dramatically in the wake of the Sept. 11, 2001, attacks but largely are hidden from public view.

In one case, FBI agents kept an unidentified target under surveillance for at least five years - including more than 15 months without notifying Justice Department lawyers after the subject had moved from New York to Detroit. An FBI investigation concluded the delay was a violation of Justice guidelines and prevented the department "from exercising its responsibility for oversight and approval of an ongoing foreign counterintelligence investigation of a U.S. person."

In other cases, agents obtained e-mails after a warrant expired, seized bank records without proper authority and conducted an improper "unconsented physical search," according to the documents.

Although heavily censored, the documents provide a rare glimpse into the world of domestic spying, which is governed by a secret court and overseen by a presidential board that does not publicize its deliberations. The records also emerge as the House and Senate battle over whether to put new restrictions on the USA Patriot Act, which made it easier for the government to conduct secret searches and surveillance but has come under attack from civil-liberties groups.

The records were provided to The Washington Post by the Electronic Privacy Information Center, an advocacy group that has sued the Justice Department for records relating to the Patriot Act.

David Sobel, the center's general counsel, said the new documents raise questions about the extent of possible misconduct in counterintelligence investigations and underscore the need for greater congressional oversight of clandestine surveillance within the U.S.

"We're seeing what might be the tip of the iceberg at the FBI and across the intelligence community," Sobel said. "It indicates that the existing mechanisms do not appear adequate to prevent abuses or to ensure the public that abuses that are identified are treated seriously and remedied."

FBI officials disagreed, saying that none of the cases has involved major violations and most amount to administrative errors. The officials also said any information obtained from improper searches or eavesdropping is quarantined and eventually destroyed.

"Every investigator wants to make sure that their investigation is handled appropriately because they're not going to be allowed to keep information that they didn't have the proper authority to obtain," said one senior FBI official, who declined to be identified by name because of the ongoing litigation.

The documents provided to the center focus on 13 cases from 2002 to 2004 that were referred to the Intelligence Oversight Board, an arm of the President's Foreign Intelligence Advisory Board that is charged with examining violations of the laws and directives governing clandestine surveillance. Case numbers on the documents indicate a minimum of 287 potential violations were identified by the FBI during those three years, but the actual number is higher because the records are incomplete.

FBI officials declined to say how many violations they have identified or how many were found to be serious enough to refer to the oversight board.

Catherine Lotrionte, the presidential board's counsel, said most of its work is classified and covered by executive privilege. The board's investigations range include "technical violations to more substantive violations of statutes or executive orders," Lotrionte said.

In a letter to be sent today to the Senate Judiciary Committee, Sobel and other Electronic Privacy Information Center officials say the documents show how little Congress and the public know about the use of clandestine surveillance by the FBI and other agencies. The group advocates legislation requiring the attorney general to report violations to the Senate.