Thomas Reppetto, a former Chicago police officer who became a respected historian of policing and the leader of a nonpartisan watchdog group that researches ways to reduce crime in New York City, died on Tuesday at his home in Mount Vernon, N.Y. He was 88.

His wife, Christa Carnegie, said the cause was congestive heart failure.

Dr. Reppetto brought a street cop’s experience and a scholarly perspective to the Citizens Crime Commission of New York City, a small, business-funded organization, which he joined as president at its inception in 1979. He quickly became a prominent advocate for solutions to criminal justice problems like the need for more police officers and stemming the sale of illegal firearms.

He was also a go-to expert when the news media needed a Harvard-educated former officer to discuss crime and policing.

“Tom was an extraordinarily strong advocate for the importance of policing — but appropriate policing,” William J. Bratton, a former New York City police commissioner, said in an interview. “He loved being a cop, he loved being the head of the commission, and he was cleareyed about the excitement as well as the flaws of the profession.”

Dr. Reppetto did not see the criminal justice system as a failure at deterring crime, but he said that a “major policy error” had been made after World War II when police departments, for efficiency, moved officers off their beats and into patrol cars.

“In this role, the officer was seen as a crime fighter, not an arbiter or peacekeeper, and in theory this made sense,” he said at a panel discussion organized by the commission in 1985. “But in practice, it allowed disorder to grow in the streets of American cities, and the police lost contact with the public, particularly young people.”

In 1990 — when there were a record 2,245 murders in New York City — he proposed that Mayor David N. Dinkins hire 5,000 police officers, to be paid through a special tax and other revenues, as a way to return the force to grass-roots patrolling.

(Over the next few years, partly at the commission’s urging, at least 5,000 officers were added to the department, using property taxes and a personal income tax surcharge.)

Even as crime fell through the 1990s, Dr. Reppetto remained wary of any proposed reductions of officers at the New York Police Department and cautioned against complacency.

“People take for granted that crime is going down, but nothing should be taken for granted in law enforcement,” he told The New York Times in 2001. “In policing, things go from bad to good — and from good to bad — very rapidly.”

ImageDr. Reppetto wrote extensively about policing and organized crime.

By 2005, when Dr. Reppetto left the commission, he had already begun to write books about policing and organized crime. With James Lardner, he wrote “NYPD: A City and Its Police” (2000); on his own, he wrote “American Mafia: A History of Its Rise to Power” (2004) and a sequel, “Bringing Down the Mob: The War Against the American Mafia” (2006).

Reviewing the first Mafia book for The Times, Dan Barry wrote, “Though the bookshelves cry for mercy under the weight of Mafia literature, Reppetto’s book earns its place among the best, in part because he rarely lapses into belly-full-of-lead prose.”

Dr. Reppetto also wrote the history “American Police,” published in two volumes in 2010 and 2012, and “American Detective: Behind the Scenes of Famous Criminal Investigations” (2018).

Thomas Anthony Reppetto was born on Aug. 17, 1931, in Chicago. His mother, June (Blakely) Reppetto, was a secretary at the Chicago Police Department. His father, George — nicknamed Peanuts — was a saloonkeeper with a rap sheet from bookmaking and other criminal activities. Father and son bonded on Sundays when Tom visited his father in prison.

Dr. Reppetto graduated from Roosevelt University with a bachelor’s degree in political science and joined the Chicago Police Department in 1952. He rose to captain and, finally, commander of detectives. Near the end of his 18 years with the department, he began to study for a doctorate in public administration at Harvard, hoping the added education would help his police work.

But Ms. Carnegie, his wife, said in an interview that after he returned full time to the force in 1970, “they were suspicious of him and put him in some planning department, which he quit two months later.”

“I don’t know what happened,” she added. “There was a lot of politics.”

Credit…Jack Manning/The New York Times

He left Chicago in 1970 to do criminal justice research at the Joint Center for Urban Studies of M.I.T. and Harvard (now the Joint Center for Housing Studies of Harvard) before moving to New York City the next year to teach at the John Jay College of Criminal Justice in Manhattan. In 1977 he was named the school’s dean of graduate studies; a year later, he was named vice president.

In 1979, a group of businessmen, concerned about the effectiveness of the city’s criminal justice system, created the independent Citizens Crime Commission to monitor the performance of the police and correction departments, as well as prosecutors and the courts, through reports and public forums. Dr. Reppetto was hired to run the commission, which had a small budget and only a few staff members.

Richard Aborn, the current president of the commission, said that Dr. Reppetto “brought the ability to understand crime and crime patterns” and “was able to conduct research that provided actionable insights.”

In addition to his wife, Dr. Reppetto is survived by a daughter, Martha Reppetto. His marriage to Cecelia Seibert ended in divorce.

Mr. Bratton recalled that Dr. Reppetto brought together swaths of the city’s criminal justice system — judges, police commissioners and prosecutors — to the commission’s monthly breakfasts and lunches. At times, he said, he had to playfully referee good-natured arguments between the equally opinionated Dr. Reppetto and Jack Maple, a leading police strategist who served as deputy police commissioner under Mr. Bratton.

“They were must-see and must-be-seen-at events,” said Mr. Bratton, who began attending the meetings in 1990 when he took over the city’s transit police after serving as superintendent of Boston’s Metropolitan Police. “To come here, as an outsider, and find a place like that to interact with these Damon Runyon characters was so important. No place in America had anything like it.”