Reputed mobster William P. Masselli tried to cooperate with federal investigators last May and establish that he had paid a $20,000 kickback to an official of Secretary of Labor Raymond J. Donovan's construction firm in return for a $200,000 loan.

Special prosecutor Leon Silverman and his investigators did not find sufficient corroboration for the charge but had been hoping that Masselli and his son, Nat, could help them pursue fresh allegations concerning Donovan that prompted them to reopen their inquiry this summer.

Nat Masselli, 31, was shot to death in a gang-land-style slaying in the Bronx last week less than 48 hours before his father was scheduled to be called before a federal grand jury working under Silverman's supervision.

At the time of his statements to Silverman last May, the elder Masselli, 56, a reputed member of the Genovese family of the Mafia, had concluded an unsuccessful effort to win a court-ordered reduction in a seven-year prison sentence.

Before going to prison, Masselli had told FBI agents last Jan. 7 "that there were no loans" from Donovan's firm, the Schiavone Construction Co., to him or his firm, Jopel Construction and Trucking, a Schiavone subcontractor. Masselli's lawyer amended that to say that Jopel had gotten $200,000 in "start-up" money from Schiavone for equipment needs but the money had been repaid.

On May 17, however, Masselli told Silverman that "in exchange for a $200,000 loan from Schiavone , Masselli paid Schiavone vice president Joseph DiCarolis $20,000 in cash."

Masselli also apparently produced a piece of notepaper imprinted "From the Desk of Joseph DiCarolis." Masselli said DiCarolis had given it to him around August of 1981 in midtown Manhattan at or near the offices for Schiavone's 63rd Street subway project.

Silverman said earlier in the investigation that he had been unable to obtain a copy of the document, which had various numbers jotted on it.

"According to Masselli, DiCarolis used the paper to explain how much money Masselli had previously given to DiCarolis and how much DiCarolis believed he still had coming," Silverman subsequently reported. "In essence, DiCarolis was dunning Masselli for money."

The first line of the document shows a "$200,000 loan" under a handwritten heading for "Jopel" and then on the same line lists "20,000" as "Received."

DiCarolis was interviewed by Silverman's office on Jan. 15, 1982, before the document became available. He told investigators he first met Masselli in 1976 when he took over a Schiavone subcontract in Queens for excavation work. When Masselli's company later became the subcontractor on the 63rd Street project, DiCarolis said that "Masselli informed DiCarolis that Jopel needed money for trucks."

According to DiCarolis, "that was the genesis of the $200,000 loan." DiCarolis said he made it "entirely on his authority, speaking neither to Ronald Schiavone or Raymond Donovan about the matter." DiCarolis also stated that "he never received any kickbacks from Masselli or anybody else."

As for the "DiCarolis paper," which Silverman had by then heard about, DiCarolis said he wrote it down at one point to counter complaints from Masselli about expenses on the 63rd Street job. DiCarolis suggested that the $20,000 reflected the "savings" to Jopel in not having to pay interest on the $200,000 loan.

The $200,000 Schiavone loan check itself was made out on March 13, 1979, and signed by both DiCarolis and Raymond Donovan, then executive vice president. It was repaid by the end of September by means of deductions from the monies due Jopel each month for its work.

When Donovan was interviewed about the matter on May 10, 1982, the labor secretary said "he was not privy" to the discussions about the loan and did not recall signing the $200,000 check. He said he had "never heard of anyone at SCC, including DiCarolis, refer to any $20,000 kickback on the $200,000 loan."

In the current phase of the investigation, Silverman evidently hoped that the Massellis might throw some light on reports that Donovan met in Miami in January, 1979, with Masselli and gangland figure Albert (Chink) Facchiano.

Facchiano and Masselli are in a New York prison waiting to be called before Silverman's panel.

Silverman questioned Donovan about the latest allegations for about 2 1/2 hours Sunday in New York. "I am fully confident that Mr. Silverman's report will be as favorable as the previous one . . . that I will be fully cleared of the newest allegations," Donovan said in a statement last night.