Jeff Katz, who has died aged 74 from a heart attack, played a key role in changes to the corporate investigation business, first at Kroll Associates, then at Bishop International.

Long before the Leveson inquiry, and as far back as the early 1970s, there have been prosecutions in the UK for phone hacking and blagging of information involving small-time private investigators, some of them working on behalf of the press, and colourful cases of industrial espionage in the US involving so-called “dumpster diving” by PIs acting for corporate clients. As a former journalist Katz was well aware of this history and looked to move away from such dubious tactics and players.

He was an advocate of government regulation of the £250m-a-year business that has emerged in Britain over the past four decades. Rather than the world-weary private eyes of film and literature, corporate investigation involves expensive organisations dominated by lawyers, accountants, former MI6/MI5 agents and retired senior police or military officers. They prefer to be known as business intelligence providers, strategic advisers and risk or security consultants, relying on public record and legally obtained information.

Katz’s highest-profile case was a two-year Kroll investigation that he led into the death in London of the Italian banker Roberto Calvi, which undermined the official version that Calvi had hanged himself beneath Blackfriars Bridge in 1982. His family and Italian prosecutors suspected Calvi – known as “God’s banker” for the connections between Banco Ambrosiano, of which he was chairman, and the Vatican – had been murdered by the Sicilian mafia over the bank’s losses and to ensure his silence about money laundering and the secret P2 masonic lodge, part of a “deep state” network of businessmen, politicians, securocrats and mobsters.

Roberto Calvi, an Italian banker whose death was investigated by Jeff Katz at Kroll Associates after he was found hanging under Blackfriars Bridge, London, in 1982. Photograph: AP

Corporate investigation is most often into tracing assets, not disproving suicide. “The issue for us was usually money – not death,” Katz later wrote. A detailed forensic reconstruction, in which an actor climbed on to the original scaffolding under the bridge wearing a pair of the same shoes Calvi had worn, which were then immersed in water, suggested that Calvi could not have walked on the scaffolding. It seemed more likely he had been murdered and placed there by boat.

Mafia informants later stated Calvi had been strangled. However, the individuals identified in the investigation were acquitted of murder after a trial in Rome in 2007 on grounds of “insufficient proof”.

Less well known were Katz’s efforts to trace Robert Levinson, a family friend, former FBI agent and sometime CIA contractor, who disappeared after travelling to Kish Island, off the coast of Iran, in March 2007. Levinson worked for Bishop investigating cigarette smuggling, and had used this cover for travelling to Kish, a visit encouraged by CIA officials. Last seen alive in hostage photographs in 2011, Levinson was suspected to have been held by Iran’s Revolutionary Guard. Katz made attempts to elicit information from inside Iran on behalf of the family. Despite these and official US efforts Levinson was never traced. In early 2020 the US government stated that it believed Levinson was dead, and in December it named two senior Iranian intelligence officials as responsible for his detention. Iran denied involvement.

Another of Katz’s cases concerned the British student Jeremiah Duggan, who was found dead beside a German motorway in 2003 while attending a conference of the far-right extremist organisation headed by the American Lyndon LaRouche. The German police concluded Duggan had killed himself by running into the traffic, but his family believed he had been murdered. His mother, Erica, had received disturbing telephone calls shortly before his death. Katz traced a former LaRouche follower who had useful information. “He opened up the case for me,” said Erica. A London coroner in 2015 rejected the German police verdict of suicide, pointing to a number of unexplained injuries. The German investigation was closed in 2018.

A Bishop investigation led by Katz persuaded the Kenyan authorities in 2017 to drop murder charges against a British executive, Richard Alden. Forensic evidence indicated that the Kenyan woman he was accused of killing had shot herself in a tragic accident.

The son of Max, a construction worker, and Mollie (nee Portugalo), who worked at a department store, Katz was born in New York. His interest in journalism began at DeWitt Clinton high school in the Bronx, where he was editor of its award-winning newspaper. In 1962 he dropped out of college to become a copy boy at the New York Times and recalled delivering the news that President Kennedy was dead the following year.

Katz travelled to Britain in 1969, serving with the US Air Force at RAF Chicksands near Bedford as an information specialist. It was “the closest job I could get to being a journalist”, he said. Katz worked for Time Out magazine before returning to New York in 1973, and graduating from the City University of New York in 1975 after studying English literature and film production.

In 1979 he returned to Britain to join Bedfordshire on Sunday, a free paper that, under the editorship of Frank Branston, developed a national reputation for exposing local scandals. Katz worked as a reporter, arts reviewer and keen photographer.

The switch to corporate investigator in 1987 followed an invitation from a family friend in New York who worked for Kroll Associates, which had opened a London office. The agency’s founder, the former New York prosecutor Jules Kroll, is seen as the father of the corporate investigation business. A Kroll diaspora created a succession of competitors.

Katz rose to be head of European operations. He left Kroll in 1998 and a year later became chief executive of Bishop International, which specialised in insurance investigations run by the traditional mix of former police officers. Katz revived Bishop by recruiting from the Serious Fraud Office and the Serious Organised Crime Agency, as well as taking on intellectual property experts, to add to the usual diet of due diligence, asset tracing and litigation support. “Always hire people who are smarter than you,” he advised.

Working for clients whose instructions were “spend whatever it takes”, Katz was equally generous with his time assisting the training of less well-resourced journalists. He organised a detailed guide to offshore tax havens. “There is no such thing as a stupid question, just stupid answers,” was his rule.

Driven by client demand, the dark side of the corporate investigation business still thrives, but it is mainly provided now from covert sources beyond UK jurisdiction. Katz supported more effective oversight of the business in evidence given to a Commons home affairs committee in 2013, but that has not been implemented.

Katz is survived by his partner, Frances Wilkinson, and her son, Matt, and by his brother, David.

• Jeffrey Morris Katz, corporate investigator, born 10 April 1946; died 9 December 2020