(This article contains material adapted from “Missing Man: The American Spy Who Vanished in Iran,” by Barry Meier, published May 3 by Farrar, Straus & Giroux. )

In early March, a small group of private investigators, including two former FBI agents, gathered for a meal at Old Tbilisi Garden, a restaurant in Greenwich Village that specializes in Georgian food.

It was a somber occasion. Two months earlier, the United States and Iran had exchanged prisoners, including several Americans held in Tehran’s notorious Evin Prison. Another American, Robert A. Levinson, long missing in Iran and a friend of those present, was not part of the deal. Levinson, a former FBI agent who became a private investigator, also had another life: as a consultant for the CIA.

In March 2007, Levinson, then 59, disappeared on Kish Island, in the Persian Gulf off the coast of Iran, while trying to recruit a fugitive U.S.-born assassin as a CIA source inside Iran. He was last seen alive in 2010 in a hostage video pleading for help and in photographs wearing a Guantánamo-style jumpsuit. The images did not disclose who was holding him. It is not known whether Levinson, who was eager to expand his role at the CIA and who apparently decided on his own to go to Iran, is still alive.

The event at Old Tbilisi was held to observe the ninth anniversary of his disappearance. Former colleagues toasted him and traded stories. Levinson, like those present, had spent his career in a shadow land, one where spies, agents and private eyes persuade informants to spill secrets in exchange for money or a deal. It is a risky game that can pay off big or that can go very wrong, as Levinson’s trip to Iran did.

The two former FBI agents at the dinner, Joseph F. O’Brien and Andris Kurins, were familiar with that terrain. O’Brien had spearheaded the FBI’s investigation in the mid-1980s of Paul Castellano, the reputed head of the Gambino crime family who was gunned down outside Sparks Steak House on East 46th Street.

After Levinson’s disappearance, the former agents had tried to help the FBI find their missing friend. O’Brien recruited a former Iranian army general to go to Tehran to seek out information. But as with other investigators who knew Levinson, their offers were rebuffed by the FBI.

“I was told, ‘Why don’t you mind your business?’” recalled Kurins, who worked with O’Brien on the Castellano case.

Levinson and his wife, Christine, lived for years in Coral Springs, Florida, and raised their seven children there, but he was a New Yorker in every other way. He grew up on Long Island, attended City College, met his future wife at a quintessential Manhattan singles bar, T.G.I. Friday’s, and worked in New York on celebrated cases both as an FBI agent and as a private investigator.

Before his disappearance, Levinson also frequently came to New York to meet with clients or to try to hustle up new assignments. James Mintz, who heads a private investigation firm, said Levinson came by his Manhattan office not long before he vanished to see if he had any cases involving criminals trafficking in counterfeit cigarettes, an area in which he specialized. Levinson also worked for a major corporate investigation firm, SafirRosetti, whose clients included The New York Times.

Levinson, known to his friends as Bob, liked to swap information with journalists interested in espionage and intrigue. Some of his tips paid off; others were wild goose chases. Brian Ross, the chief investigative correspondent for ABC News, who knew Levinson for decades, said he turned up at ABC headquarters before his disappearance with what sounded like a major scoop.

Levinson told him his sources had located a major al-Qaida operative in Venezuela and were observing the man going into a local mosque twice a day. ABC News dispatched an employee to Venezuela, where he rented a room at a hotel across the street from the mosque and waited for the terrorist to appear.

“We spent a week there staking it out,” Ross said not long ago. “Nothing happened.”

Levinson told his children that he knew from the age of 8 that he wanted to be an FBI agent. His epiphany came, he said, while watching a movie called “The House on 92nd Street,” a low-budget thriller about a college student who goes undercover for the FBI during World War II to break up a Nazi spy ring.

As a teenager, Levinson liked to hang out with friends in the attic of his family’s home in New Hyde Park acting out courtroom dramas. For dialogue, they used transcripts from real trials typed up by his mother, who was a part-time court stenographer.

From the start of his career, Levinson saw himself as a collector of informants, someone skilled in extracting information from people eager to catch a break from the law or in need of a favor, like a U.S. visa. By the late 1970s, he had his dream job, working in the FBI’s New York office, helping to coordinate intelligence about the Mafia gathered by agents in the metropolitan area.

O’Brien, who also wanted to investigate the mob, knew he had found his mentor when he first met with Levinson. His office, O’Brien recalled, was lined with photos of gangsters and charts showing the hierarchies of New York’s major crime families. “He was a master, and he taught me a lot,” O’Brien said.

In 1983, the two men shared an FBI achievement award, and not long afterward, O’Brien arrested Castellano. But Levinson’s FBI career nearly ended over his handling of an informant in another big Mafia case — one that would eventually involve President Reagan’s secretary of labor, Raymond J. Donovan.

His source was a onetime schoolteacher named Michael Orlando, who had turned to robbery. When faced with the prospect of more jail time, he agreed to cooperate with the FBI as a paid informant on another local crime family, the Genoveses.

Levinson believed that Orlando was critical to building a sweeping case that would result in indictments against politicians, labor leaders and police officers. But some bureau officials suspected that the FBI agent had fallen into the classic informant’s trap, with Orlando feeding him tidbits of information while using his protected status to carry out crimes. One member of the Genovese family was even heard on an FBI wiretap discussing how Orlando had become a contract killer.

Levinson and other agents fought to keep using Orlando. But when FBI supervisors decided to arrest him and end the investigation, a bitter dispute broke out inside the FBI that cost Levinson his position in New York and led to his decision to move to Florida.

There, he became an expert in the FBI’s Miami office on Colombian drug cartels and Russian organized crime. After his retirement in 1998, he took a familiar path, followed by many former agents, into the private sector, working for corporate investigation firms and setting up his own one-man shop.

Investigators like Kurins said they did not believe the story that the U.S. government had put out to explain why Levinson was in Iran — that he had gone there to track counterfeit cigarettes.

“It was impossible that he would go to Kish Island on a cigarette case,” Kurins said.

Several months after Levinson’s disappearance, another of his acquaintances in New York, John Moscow, a lawyer, checked his email and found what appeared to be a clue. Moscow, a former top prosecutor in the Manhattan District Attorney’s Office, was one of several people copied on an email in which Levinson, or someone pretending to be him, pleaded for help from U.S. officials.

Moscow said he had turned the email over to the FBI, adding that he had never heard back from bureau officials. “They told me they would handle it,” he said.

FBI officials told Levinson’s family that they thought the email was a fake. But three years later, when an email arrived containing a video showing him as a hostage, it seemed that the bureau may have made a critical mistake. The 2007 email received by Moscow and the 2010 message with the video were sent from Gmail accounts bearing nearly identical names, according to a new book about Levinson and the search for him.

A spokeswoman for the FBI, Lindsay Ram, declined to comment.

Over time, the FBI employed an unusual cast of characters to search for Levinson, including a billionaire Russian oligarch, Oleg Deripaska. Frustrated by the lack of progress, O’Brien decided to take action.

He first tried to hire a researcher in Tehran to search through newspapers and public records for any mention of Levinson. That approach never got off the ground. In 2009, two FBI agents based in Washington called and said they wanted to see him.

O’Brien said he met the agents at the Gracie Mews Diner on the Upper East Side. The men initially stood by the government’s explanation for Levinson’s presence in Iran. But when O’Brien pressed his visitors, they began to open up to him.

The FBI agents told him there was infighting at the CIA about how to react publicly and privately to Levinson’s disappearance.

“This was going to embarrass the CIA,” he said, “and these agents didn’t feel like the agency was telling them the truth.”

The agents also told him, he added, that the National Security Council was considering issuing an official protest accusing Iran of holding Levinson as a hostage, a step the Obama administration never took. Ram, the FBI spokeswoman, declined to comment, as did a spokeswoman for the security council, Dew Tiantawach.

In 2012, however, O’Brien saw an opening. Earlier, he had urged one of his neighbors at his weekend place in the Poconos, a former Iranian army general, to ask about Levinson during a trip home. The man had reported back that his best friend there, a well-connected Iranian general, had been afraid to talk. But O’Brien was soon able to use a tactic familiar to Levinson: turning a source’s misfortune to his favor.

When O’Brien visited his Iranian-American friend to wish him a happy Father’s Day, he found the man distraught because the FBI had just arrested his son on fraud charges. Sensing a deal, O’Brien offered to seek a reduced sentence for his son from the Justice Department if he went back to Iran and told his former colleague there that he needed information about Levinson to save his child.

The man agreed and went back to Iran, where he awaited a green light, telling him a deal had been struck. But that signal was never sent, O’Brien said, because everyone he contacted at the Justice Department and the FBI threw up a roadblock.

“We have tried to make deals, and they have not produced anything credible,” one FBI official told him, he said.

The Obama administration has said it is committed to finding Levinson. O’Brien said that he and Kurins planned to dedicate the follow-up to their 1991 best seller about the Castellano case, “Boss of Bosses,” to Levinson’s wife and their children. (Both O’Brien and Kurins resigned from the FBI amid controversy over that book.) But O’Brien will never accept the fact, he said, that the American government did not insist upon Levinson’s return — or at least information about his fate — as part of the prisoner exchange early this year.

“I don’t know what these other people were doing over there,” he remarked. “But they weren’t patriots working for their government.”

© 2016 The New York Times Company