In an unusual memorandum sent to the United States attorney in Manhattan, the investigator, Walter Mack, also called on prosecutors to investigate why the carpenters grew more lax in enforcing safety and union rules at the site where Mr. Gotti worked.
Mr. Mack also urged prosecutors to investigate Silo Construction, the company that employed Mr. Gotti, noting that one shop steward had complained about being threatened if he continued filing safety complaints about the construction site.
Mr. Mack, a former federal prosecutor who stepped down as the union's investigator last month, wrote that the shop steward, Peter O'Keefe, had helped Silo defraud the union by failing to supply reports on weekend work. That, Mr. Mack suggested, allowed the company to avoid making pension, health and other benefit payments for that work.
Since 1994, the union, the New York District Council of the United Brotherhood of Carpenters and Joiners, has been operating under a federal consent decree as the government has sought to wipe out corruption and mob influence in that union.
Officials with the district council, which represents 25,000 carpenters, say government and internal monitoring efforts have largely eliminated corruption.
Gary Rothman, a lawyer for the district council, said that with the assistance of Mr. Mack, his successor and outside lawyers, "the union is trying to respond to and stamp out vestiges of organized crime influence in the council."
In the 1990's, federal prosecutors said, the Genovese crime family had considerable influence over the carpenters, while John Gotti, the Gambino crime boss, exerted a heavy hand over several Teamster and mason tender locals.
Mr. Mack originally sent the memorandum, titled "Recommendation for Criminal Referral," to the United States attorney, to the union and to Judge Charles S. Haight Jr. of United States District Court in Manhattan, who oversees the federal consent decree with the carpenters. Mr. Mack sent it as a confidential memorandum in late June, but Judge Haight unsealed it in late August.
Bridget Kelly, a spokeswoman for the United States attorney, declined to comment on Mr. Mack's report.
Silo had no listed phone numbers. A man who answered the phone at a related company yesterday said that Silo's principals were not reachable. Mr. Rothman, the union's lawyer, said Silo had gone out of business after some judgments against it.
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Mr. Mack said Mr. O'Keefe had testified in a session with investigators that in 2002, halfway through the project, Silo had told him to include Mr. Gotti on the list of union carpenters, even though Mr. Gotti had a criminal record.
On the basis of testimony from Mr. O'Keefe, Mr. Mack wrote that Mr. Gotti told Mr. O'Keefe "that if there were any further safety issues they would work themselves out and that O'Keefe should not report them to the union."
Mr. Mack further wrote: "Raising his voice and touching O'Keefe's chest with his finger, Gotti said that if O'Keefe did make further reports to the union, it would be he who would have the problem. O'Keefe testified that he was intimidated by Gotti and did not sleep well for a few nights."
Responding to Mr. Mack's criticisms of the union's failure to stop the hiring of Mr. Gotti as a carpenter, Mr. Rothman said, "The union doesn't run criminal background checks for people who apply for membership in the union."
He suggested that top union officials did not know about Mr. Gotti's membership in the union because, he said, Mr. Gotti's name appeared on only two or three of the weekly shop steward's reports of workers that were supplied to headquarters.
Mr. Rothman noted that the union had suspended Mr. O'Keefe from working as a shop steward for failing to report the use of nonunion workers. Mr. O'Keefe denied any wrongdoing on that issue.
In his report, Mr. Mack refers to one union business agent who investigated safety problems, but "did not want to come to the site because he was afraid that he would be thrown off the building."
At the time that Mr. Mack wrote the report, tensions existed between him and the carpenters' district council. Mr. Mack complained that he was being pushed out as investigator for being too aggressive. He had suggested that the district council was not being vigorous enough in cracking down on mob influence and employers who sought to defraud the union by, for instance, using nonunion workers at union sites.
Denying that Mr. Mack had been pushed out, Mr. Rothman said his two-year term had simply ended.
Michael Forde, then the president of the District Council of Carpenters, was convicted last year of taking payoffs to allow the use of nonunion labor. In May, a Manhattan judge threw out the conviction because of juror misconduct.