FBI agents, in a three-week investigation of loans from the National Bank of Georgia to the Carter peanut warehouse in Plains, have found indications that the bank's records may have been falsified, The Washington Post has learned.

Sources say that the FBI investigation raised the question of whether Billy Carter, the president's brother, and at least one bank official were involved in doctoring the records.

Late last night Justice Department spokesman Terry Adamson said: "The report received from the FBI last week contains no allegations whatsoever about falsification of records or doctoring of records."

The suspected manipulation of the Carter warehouse loan records, which went undetected both by bank and warehouse accountants, was to show there was more collateral backing the loans than existed, sources say.

The sources stress that the report was prepared in a rush and that findings are preliminary.

The FBI investigation of the loans was an offshoot of a federal grand jury probe in Atlanta of the financial affairs of former federal budget director Bert Lance.

Lance arranged the loans in 1975, shortly after he became president of NBG. The line of credit, which quickly grew to $6.5 million, became the bank's biggest account.

FBI agents assigned to the investigation think that the questionable handling of the loans in 1976 and 1977 was directed by Lance and carried out by Robert Flynt, then an NBG loan officer, sources say.

Flynt, now with a bank in Jacksonville, Fla., refused to comment yesterday on the report. "I couldn't comment. I'll refer that to NBG," he said.

"It was not necessarily me," he said about the FBI report concerning possible alteration of bank records.

Billy Carter was in Sumter County, Ga., Hospital where he was admitted on Friday for treatment of bronchitis. An attorney in the Americus law firm representing Carter said there would be no comment on the report.

One of Lance's attorneys, Robert Altman, said: "I have discussed at length with Mr. Lance and others during the course of representing him his activities at NBG and would flatly deny any allegation that he had any knowledge of any falsification of records at the bank."

In December, Billy Carter testified before the Atlanta grand jury investigating Lance's finances. Later, he told reporters that he had invoked the Fifth Amendment against self-incrimination on certain questions asked by federal attorneys.

This week, according to sources, that same grand jury will take up the findings of the FBI probe into the Carter warehouse loans.

The grand jury will also continue to hear testimony on Lance's affairs. Members of the boards of directors of NBG and Calhoun, Ga., First National Bank, which Lance also headed, will be called to testify, some of them for a second time, sources say.

Billy Carter took over the running of the warehouse in 1975 when Jimmy Carter began his run for the presidency. In mid-1977, when Charles Kirbo, the president's trustee, refused his offer to buy the business, Billy bowed out of his management role. A professional management company took over the operation.

Richard Beckler, the acting chief of the Justice Department's fraud section who is in charge of the Carter warehouse probe, refused to comment on the FBI report.

The FBI's findings are contained in a two-inch-thick report prepared in just three weeks, sources say. A team of about 12 agents drawn from several bureaus around the country carried out the investigation.

The FBI report comes as auditors from the Internal Revenue Service are winding up their own review of the Carter family peanut business. According to a Treasury source, the IRS agents found the 1975 warehouse records in order.

The same source says the IRS found "only an $84 arithmetical error" in Billy Carter's 1975 personal returns. In fact, says the source, the records were so accurate that only a cursory "survey" of the Carter warehouse's 1976 tax return and Billy Carter's 1976 return was deemed necessary.

Morover, the Treasury source was critical of the FBI for not availing themselves of several thousand pages of IRS findings on the Carter warehouse finances. He added that some records the FBI is now seeking have been thrown away.

In answer, a source knowledgeable about the FBI findings says the IRS probe was far narrower. "The IRS viewed everything only from an income standpoint," he said.

The FBI's investigation of the Carter warehouse loans leaves officials at the Justice Department with some difficult decisions.

Are the findings criminal in nature? If so, should the investigation be handled by the Justice Department through the grand jury or should a special prosecutor be appointed?

A recently enacted law provides for the appointment of a special prosecutor when sufficiently serious allegations are made against an officeholder or a former officeholder. The law does not apply to family or friends of an officeholder. But the attorney general on his own can appoint a special prosecutor to investigate cases that are politically troublesome.

The Carter warehouse operation has been having financial problems in recent years. Kirbo, who is currently overseeing the business, has said that the operation is up for sale.

The business, begun by Lillian Carter's late husband, is owned 15 percent by Billy Carter, 63 percent by Jimmy and 22 percent by their mother.

Kirbo suggested that the warehouse's troubles have been caused by a combination of poor peanut crops, management problems and a costly expansion program.

Of the $6.5 million borrowed from NBG, about $1 million was used to construct a peanut sheller and to build a new warehouse.

The balance of the loan was used to help finance the purchase of peanuts from the farmers and storage of the peanuts until they could be sold to processors.

Kirbo has claimed that Billy got into trouble when he took IOUs from farmers who were buying seed and other supplies at the warehouse. The farmers promised to deliver their peanut crop to the Carter warehouse.

These accounts receivable from the farmers were pledged as collateral on the NBG loans. But because of a drought in the spring of 1977, some of the farmers harvested only about half the expected peanuts; others had their crops wiped out.

According to a recent report by NBG's outside directors, prepared as part of the settlement terms of a Securities and Exchange Commission suit last April, Billy's NBG account was often overdrawn in repaying the warehouse's credit line.

The report said Lance instituted a policy at the bank that the Carter checks would not be bounced. Instead, they were held until sufficient funds were raised to cover overdrafts.

The FBI agents, who had the power to subpoena records and individuals, conducted a far broader probe on the Carter loans than have others so far, according to sources.