Federal prosecutors conducting a two-year probe of federal and D.C. contracts recently offered to plea bargain with contractor John B. Clyburn, a key subject of the inquiry, but Clyburn turned down the invitation and apparently plans to fight any charges in court, according to sources familiar with the probe.

T. Conrad Monts, a local developer and another subject of the probe, also has declined to cooperate with prosecutors, the sources said. The refusal of the men to cooperate may dampen the prosecutors' hopes of advancing the contracts inquiry beyond already identified subjects, the sources said.

Clyburn, who has been under investigation in connection with contracts awarded to him by several D.C. agencies and the U.S. Department of Housing and Urban Development, has been informed that prosecutors will seek an indictment against him within three months, the sources said.

The prosecutors also have told Monts they plan to bring charges against him within the same period in connection with a federal contract, according to the sources. No details of that contract could be learned yesterday.

A May 1987 search warrant for the office of Clyburn's former firm, Decisions Information Systems Corp., stated that investigators had "probable cause to believe" that Clyburn was guilty of racketeering, bribery, fraud and other crimes.

Thomas R. Dyson, who represents Clyburn, said yesterday that his client "is not plea bargaining, and if indicted, he will defend himself." Dyson said Clyburn "has been vilified and prosecuted in the news media on a constant basis since May. Yet Mr. Clyburn has yet to be formally charged or indicted for any crime."

"He never bid on or sought to obtain a contract that he wasn't qualified to perform," Dyson said. "He hasn't violated any laws. He hasn't paid anybody to obtain contracts. Every D.C. contract and federal government contract has been satisfactorily performed."

R. Kenneth Mundy, who represents Monts, declined to discuss any federal contracts awarded to Monts' company, Travenca Development Corp. But Mundy confirmed that prosecutors have informed him that the developer will not be charged in connection with any of his business dealings with the D.C. government.

Monts has attempted to obtain development rights for a hotel-office complex over I-395 in Northwest Washington and the Camp Simms site in Southeast. A May 1987 search warrant for his offices stated that investigators had probable cause to believe Monts committed bribery, fraud and other crimes. "The reason he's not being charged is there is nothing to charge him with," Mundy said. "He is as pure as the driven snow on D.C. contracts."

Mundy, who says he has a policy of not representing clients who cooperate with the authorities, also defended Robert L. Green, the former president of the University of the District of Columbia who was acquitted last month on theft and perjury charges.

Investigators assigned to the Clyburn case have been attempting to determine whether David E. Rivers, who headed the D.C. Department of Human Services, steered contracts to Clyburn's firms or to firms that hired Clyburn as a subcontractor.

Investigators have also been pursuing the theory that James E. Baugh, HUD's general deputy assistant secretary of public and Indian housing, steered federal contracts to Clyburn in return for Clyburn's employment of Baugh's wife, Veatrice.

John F. Mercer, an attorney for Rivers, said prosecutors have not contacted Mercer for several months. Jerris Leonard, who represents the Baughs, could not be reached for comment. James Baugh has said he was not involved in the award to Clyburn's firm of a $399,000 HUD contract.

Investigators looking into Monts' dealings with the D.C. government focused on a proposal Monts submitted to the city Department of Housing and Community Development to acquire the air rights for the hotel-office complex above I-395 just south of Massachusetts Avenue NW.

Federal authorities were investigating allegations that Monts promised to guarantee a bank loan for Clyburn in exchange for Clyburn's agreement to use his influence with D.C. officials to help Monts obtain the rights. The search warrant for Monts' office also said the FBI was looking for records of Monts' unsuccessful proposal to develop the Camp Simms site, a former National Guard training facility.

The status of another inquiry into allegations that Clyburn paid hush money to Karen K. Johnson, a former associate of Mayor Marion Barry's who refused to testify before a grand jury probing drug use by city officials, could not be determined. Two sources familiar with that probe, which grew out of the contracting investigation, said prosecutors still consider it active.