Inside the F.B.I. File of Trump’s Mentor, Roy Cohn

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When the F.B.I. released a fat stack of documents about the notorious New York lawyer Roy Cohn on Friday, there were mentions of Ed Sullivan, Jimmy Hoffa, the Detroit Lions fullback Nick Pietrosante, W. Mark Felt (the F.B.I. agent better known as Deep Throat) and Morris Barney Dalitz, a Prohibition Era bootlegger known as Moe who helped turn Las Vegas into a gambling mecca.

One person whose name did not pop up: President Trump, who relied on Mr. Cohn as a lawyer, mentor and more for 13 years starting in the early 1970s, embracing his pugnacious adviser’s hit-back-harder style along the way.

[Read more about Mr. Cohn’s influence on Mr. Trump.]

The format of the release made a definitive digital search nearly impossible, but a close reading of the 748 heavily redacted pages revealed no obvious sign of Mr. Trump.

Still, the letters, memos and other papers were a reminder of the unusual realm inhabited by Mr. Cohn — friend to J. Edgar Hoover, the first director of the Federal Bureau of Investigation, and a regular at once-legendary nightspots, the Desert Inn in Las Vegas and El Morocco in Manhattan.

Adding to the documents’ relevance was their emergence on the heels of a new documentary, “Where’s My Roy Cohn?,” which returns Mr. Cohn to the spotlight with a title quoting the president’s quest for someone to fight for him as fiercely as Mr. Cohn once did.

[Read The Times review of the film, and Maureen Dowd’s interview with the director.]

Mr. Cohn was a singular celebrity by the time he died of AIDS in 1986, weeks after being disbarred for ethical misconduct.

He had made his name prosecuting Julius and Ethel Rosenberg for espionage — and pressing for their execution — and acting as Senator Joseph McCarthy’s Communist-hunting right hand in the 1950s.

Later he became a feared, and fearsome, advocate for the mafia boss Anthony Salerno, the former Yankees owner George Steinbrenner and Mr. Trump, among others.

Mr. Cohn urged against settling the case, as Mr. Trump was considering.

“Tell them to go to hell and fight the thing in court,’” Mr. Trump later recalled Mr. Cohn telling him. The Trumps ultimately did settle, but not before filing a $100 million countersuit against the government.

It was the start of a relationship during which Mr. Cohn helped Mr. Trump close signature construction deals, sued the National Football League for conspiring against his client and prepared an onerous prenuptial agreement for Mr. Trump’s first wife.

Like Mr. Trump, most of Mr. Cohn’s best-known clients do not appear in the just-released documents. Mr. Salerno, known as Fat Tony, is an exception.

The context is Mr. Cohn’s complaint to William Webster, the F.B.I. director at the time, that the bureau had “planted” a 1985 article in the The New York Post about the bureau’s efforts to try to wiretap Mr. Cohn’s office to obtain information about the Mafia boss.

Mr. Webster denies in a letter to Mr. Cohn that the F.B.I. fed information to reporters, but a separate memo by a different official acknowledged that agents had conducted surveillance of Mr. Cohn’s office “to ascertain the feasibility of installing a monitoring device to intercept the conversations of Genovese boss Anthony Salerno” and a Teamsters contact.

The vast majority of the documents — around 600 pages — cover a period from 1962 to 1964 when Mr. Cohn was being prosecuted over accusations that he had fixed a grand jury to head off indictments in a stock-fraud scheme.

The documents show investigators exhaustively tracing Mr. Cohn’s movements around the United States to determine where and when he might have received a piece of the $50,000 payoff at the center of the case.

One 1962 memo tied to the case noted that “a source of our Las Vegas office has advised that Las Vegas gamblers are greatly concerned over the extreme pressure being applied by the federal government on the Nevada gambling industry.”

A high-ranking Justice Department official, the document notes, had been approached “to determine whether he would ‘trade Las Vegas’ for ‘Roy Cohn’” — an overture that was “flatly rejected.”

The offer, the memo continues, “appears to be a clear indication that the hoodlum element would trade Roy Cohn” if doing so “would relieve the investigative pressure presently being applied to the Nevada gambling industry.

Mr. Cohn was ultimately acquitted in the case, which he considered a result of a vendetta against him by Robert F. Kennedy, the attorney general, carried out by Robert M. Morgenthau, the United States attorney in Manhattan at the time.

Once the case ended, Mr. Cohn was quick to praise the F.B.I.’s work on it, according to a 1964 memo detailing a phone call he made to a top bureau official.

“The purpose of his call was to let the F.B.I. know of his opinion that our agents, who had testified for both the prosecution and the defense, had handled themselves in complete fairness and candor,” the official, Cartha D. DeLoach, wrote.

The F.B.I. files suggest that Mr. Cohn and Mr. Hoover shared a special bond.

“You are such a great institution up and down this nation,” Mr. Cohn wrote in a 1969 letter to the bureau’s longtime director, “that I hate to see you diverted or annoyed for even a minute.”

Mr. Hoover was similarly complimentary in his reply.

“Your generous comments regarding me are indeed gratifying,” he wrote in a letter that begins, “Dear Roy.”

Mr. Cohn’s praise for the F.B.I. notwithstanding, it is clear from the files that he was a consistent target of scrutiny.

A memo recounting an “often-hostile, sometimes-shouting” appearance he made on a radio show in 1975 to debate the merits of the Rosenberg case notes his involvement in “two pending cases.”

One case involved the possible insurance-fraud loss of Mr. Cohn’s yacht, Defiance, in 1973 off the coast of Florida. The second was related to an extortion investigation of a pornographic film theater in New Jersey in which Mr. Cohn held a stake.

Asked why the file had been made public, the bureau’s press office said the release was in response to a Freedom of Information Act request. Details about the request were not immediately available.