A Democratic fund-raiser involved in Senator Clinton’s 2000 campaign has offered a guilty plea to bank fraud charges and is likely to become a government witness at the upcoming federal trial of a top finance aide to Mrs. Clinton, David Rosen, court records obtained by The New York Sun show.

As part of an FBI investigation into alleged campaign finance reporting violations by Mrs. Clinton’s campaign, the mystery witness secretly taped a conversation with Mr. Rosen in September 2002 and apparently tried to elicit statements from the former Clinton staffer about financial irregularities involving an August 2000 Hollywood fund-raising event.

Allegedly inaccurate reports about that event filed with the Federal Election Commission led to Mr. Rosen’s indictment on four counts of causing false statements to be made to federal authorities. One count was dropped by a judge last month, but Mr. Rosen is scheduled to go to trial on the remaining charges at Los Angeles on May 3.

In the documents reviewed by the Sun, the name of the FBI informant is not disclosed. However, the records offer intriguing clues that suggest the mystery witness operated at the highest echelon of Democratic politics.

“The CW [confidential witness] is related to an extremely prominent and well-known political figure. It can be expected that the fact that CW was working in an undercover capacity for the FBI will become the subject of intense media attention,” prosecutor Peter Zeidenberg wrote in a November 2004 memo asking a federal magistrate to keep the relationship under wraps.

An affidavit filed by a Los Angeles-based FBI agent, David Smith, said the informant began working with the FBI in July 2002.”The CW states that s/he is active in fundraising for the Democratic Party. CW assisted in the U.S. Senate campaign of Hillary Clinton. S/he was involved in the planning of the Clinton Gala,” Mr. Smith wrote in January 2003. “The CW is the target of an FBI investigation on unrelated bank fraud charges. S/he has since signed a plea agreement, and has agreed to cooperate in this case.”

Mr. Zeidenberg declined to comment for this story. Mr. Smith referred questions to an FBI spokeswoman.

A source familiar with the case said the reference to the informant being “related to” a famous politico means there is a familial tie. The source, who asked not to be identified, said the informant is not related to President or Mrs. Clinton.

While the informant’s work for the FBI on Mr. Rosen’s case dates back to 2002, the agency continued to work closely with the witness through the end of 2004, court documents indicate. The November 2004 filing indicates the FBI was planning to use the informant in an investigation of “a prominent political figure who may be involved in illegally soliciting foreign nationals to contribute to national political campaigns.” The target of the inquiry was suspected of “funneling illegal campaign contributions from foreign nationals to individuals running for federal office.”

There is no indication in the documents whether the “prominent political figure” who may have been soliciting illegal gifts from foreigners is the same person to whom the informant is related.

The same unidentified informant was also being asked to record phone calls with targets of an investigation into alleged political corruption in Louisiana, the court filing late last year said. That scheme involved a state senator and a “fraudulent contract worth five million dollars,” prosecutors asserted.

At one point in the court papers, the informant is referred to as “he.” The remainder of the references use a gender-neutral pronoun, “s/he.”

Sources familiar with the case said Mr. Rosen’s attorney, Paul Sandler of Baltimore, has known the identity of the mystery witness for weeks. In fact, as part of constitutionally mandated pre-trial discovery, Mr. Sandler was given a copy of the FBI’s tape of the surreptitiously recorded conversation, a person with knowledge of the matter said.

Reached at his Chicago office, Mr. Rosen referred questions to Mr. Sandler. Mr. Sandler did not respond to several messages yesterday seeking comment for this story. Mrs. Clinton’s attorney, David Kendall, could not be reached for comment. In the past, he has denied any wrongdoing by the campaign.

Two men who were involved in staging the costly and star-studded August 2000 fund-raiser for Mrs. Clinton, Peter Paul and Aaron Tonken, said yesterday they were baffled by the FBI’s description of a person who was “involved in planning of the Clinton Gala” and related to a top political figure.

“I have no clue,” said Tonken, who is serving a 63-month sentence on fraud charges stemming from celebrity charity fund-raisers he organized.

Paul, a thrice-convicted felon, has sued Mr. and Mrs. Clinton, alleging the former president and first lady took more than $2 million in illegal donations, misreported them, and failed to follow through on a deal that involved Mr. Clinton coming to work for Paul’s now-defunct Internet company, Stan Lee Media.

Many of the statements the confidential witness solicited from Mr. Rosen and which are contained in the court papers do not appear particularly incriminating.

“It was exciting stuff that we did. That gala was exciting,” Mr. Rosen is quoted as saying. “Lawsuit or no lawsuit, man … that was the most amazing thing that we put together in three weeks.”

According to the excerpts of the conversation, Mr. Rosen said, “I asked [Paul] how much he spent. He told me. We reported it. It’s his, you know, people lie to me all the time. What am I suppose [sic] to do?”

The theory of the government’s case is that by underreporting the costs of the event, the campaign maximized the amount of “hard money” raised and minimized the amount of “soft money.” Under the laws then in effect, campaign officials generally preferred hard money, because it could be spent with fewer limitations.

Mr. Rosen indicated to the informant that the high event costs caused problems for the campaign. “You rarely wanna do fifty cents to raise a dollar,” he is quoted as saying. “You have to pay the percentage out of the income. So we would have to move hard to soft. Not the other way around.”

Mrs. Clinton’s campaign has claimed that the August 2000 event cost about $600,000, up from lower figures in earlier reports. In the September 2002 recorded conversation, Mr. Rosen allegedly said, “We probably spent a million, which is 400,000 more.”

The government filing also stated that Mr. Rosen said he could not be convicted because he was only a “staffer.”

The court records do not indicate whether the informant who taped Mr. Rosen was ever asked to tape Mrs. Clinton. However, Tonken, who also claims to have worked as an FBI informant in the case, told the Sun he was asked by the FBI to tape phone conversations with Mrs. Clinton. The agent who made the request, Tonken said, was Mr. Smith, the same one who described the agency’s relationship with the mystery witness.