Selwyn Raab, an investigative reporter for The New York Times and other news organizations who in exacting detail explored the Mafia’s many tentacles, and whose doggedness helped lead to the exoneration of men wrongly convicted of notorious 1960s killings, died on Tuesday in Manhattan. He was 90.

His son-in-law, Matthew Goldstein, a Times reporter, said the cause of his death, at NewYork-Presbyterian Hospital, was intestinal complications.

Though the phrase surely fit him, Mr. Raab didn’t much care to be described as an investigative journalist. Rather, he said, “I believe in enterprise and patience.” He had both qualities in abundance across a long career, whether looking into fraudulent methadone clinics, or the life sentence given to a boy who was only 14 when convicted of murder, or the Mafia’s grip on New York City school construction.

He was also the author of a number of books about the mob, including one that became the basis of the 1970s television police drama “Kojak.”

The mob had his enduring attention as far back as the 1960s, and it led to his definitive 765-page book on New York wiseguys, “Five Families: The Rise, Decline, and Resurgence of America’s Most Powerful Mafia Empires,” published in 2005. The New Yorker writer Adam Gopnik described him in a 2020 article as “the Gibbon of the New York mob.”

His prose tended to stray from elegance. But Bryan Burrough, reviewing “Five Families” for The New York Times Book Review, said that “what makes Raab so wonderful is that he eschews legend and suspect anecdotage in favor of a Joe Friday-style just-the-facts-ma’am approach.”

Mr. Raab posited that it was Charles (Lucky) Luciano who invented the modern Mafia nearly a century ago, organizing Italian criminal operations into distinct families, with a “commission” created to resolve territorial disputes and policy matters.

In addition to tried and true enterprises — drug trafficking, gambling, prostitution — Cosa Nostra control extended to much of municipal life, Mr. Raab wrote, be it garbage removal, the garment industry, unions, construction, or fish and meat markets. Despite a popular tendency to look upon gangsters as “amiable rogues,” he said, they were murderous predators and “the invisible government of New York.”

As a boy on Manhattan’s Lower East Side, where he lived almost his entire life, Mr. Raab saw the mob up close. There, he told Time magazine in 1974, he was “surrounded by the kind of legendary criminals you read about — bookmakers, con artists, Jewish and Italian gangsters.”

“I grew up with guys I later covered,” he said.

A former Times colleague, Ralph Blumenthal, said that Mr. Raab tended to be humorless but was “a demon for the facts.” He added, “When you think of the causes he adopted, they were groundbreaking.”

That was true even before Mr. Raab joined The Times, his digging having helped free men wrongly convicted of some of the New York region’s more shocking murders. One was George Whitmore Jr., who had been imprisoned for the 1963 murders of Janice Wylie and Emily Hoffert, roommates in an Upper East Side apartment — “career girls,” as the tabloids called them.

Mr. Raab, working first for the merged newspaper The New York World-Telegram and The Sun and then for NBC News and the New York public television station WNET, uncovered evidence showing that Mr. Whitmore was elsewhere on the day of those murders and had no part in an unrelated attempted rape with which he was also charged.

Mr. Whitmore said that the police had beaten him, and that he had no lawyer during the interrogation. In 1966, his case was cited by the United States Supreme Court in Miranda v. Arizona, the landmark ruling that upheld a suspect’s right to counsel.

Mr. Raab wrote a book about the case, “Justice in the Back Room.” The book became the basis for a 1973 made-for-TV movie, “The Marcus-Nelson Murders,” which served as the pilot for “Kojak,” the CBS series about a police detective, played by Telly Savalas, which ran for five years in the 1970s.

“I’m not a detective,” Mr. Raab said. “I just look for the most reasonable approach to a story.”

He joined The Times in 1974 and worked there for 26 years. Reporting for the paper, he uncovered evidence that helped free Rubin (Hurricane) Carter, the middleweight boxer who was imprisoned for 19 years in the 1966 shooting deaths of three people in a bar in Paterson, N.J.

The Carter case was another instance of police coercion and prosecutorial overreach, one that also led to the conviction of another man, John Artis. Mr. Carter, who died in 2014, became something of a folk hero, his cause championed in a 1976 Bob Dylan song, “Hurricane,” and in a 1999 film, “The Hurricane,” in which Mr. Carter was played by Denzel Washington.

Mr. Raab received many honors across the years, including the Heywood Broun Award from the New York Newspaper Guild and an Emmy for his work on “The 51st State,” a WNET program that dealt with New York City issues and on which he was a reporter and an executive producer for three years before moving to The Times.

Selwyn Norman Raab was born on June 26, 1934, in Manhattan, one of two sons of immigrant parents: William Raab, a New York bus driver born in Austria, and Berdie (Glantz) Raab, a homemaker born in Poland.

As a boy, Mr. Raab boxed in a program run by the city’s parks department. He graduated from Seward Park High School in Lower Manhattan in 1951 and from the City College of New York in 1956, with a bachelor’s degree in English. After college, he worked for The Bridgeport Sunday Herald in Connecticut (now defunct) and The Newark Star-Ledger before joining the World-Telegram staff.

On a blind date in 1962, he met a social worker named Helene Lurie. They were married on Dec. 25, 1963. Mrs. Raab, who helped her husband with his research, died in 2019. Mr. Raab is survived by his daughter, Marian, who is chief executive of Ridge Street Productions, a film-production company.

At City College, he was an editor on Observation Post, a student newspaper. He was twice suspended from classes for brief periods because of what he wrote — first for strongly resisting student government and faculty attempts to kill the newspaper, later for criticizing college administrators who had fired several professors under attack in the McCarthy era.

He recalled those days in 2009, when he received a Townsend Harris Medal, an award given by City College in memory of its founder.

His suspensions taught him a couple of things, Mr. Raab said. One was “Never seek safe harbors to avoid contentious but important issues.” The other: “Never sacrifice integrity on fundamental principles, especially if there is a clear distinction between right and wrong on vital issues.”