Italian prosecutors have placed a businessman and former journalist under investigation for allegedly masterminding a bomb attack at the home of Sigfrido Ranucci, a prominent investigative reporter.

Valter Lavitola was supposedly friends with Ranucci, the host of Report, an investigative programme aired by the state broadcaster, Rai, and is being investigated on suspicion of trying to cause mass murder.

The development comes after police last week arrested four people – allegedly commissioned by Lavitola – on suspicion of carrying out the attack last October.

Sigfrido Ranucci has for years been under police protection owing to threats made against him. Photograph: Stefano Colarieti/AP

A rudimentary but powerful bomb was detonated outside Ranucci’s home in a town close to Rome, almost destroying a car belonging to him and another owned by his daughter.

Nobody was injured in the blast, which occurred shortly after Ranucci, who for years has been under police protection owing to threats made against him, had returned home at night.

Ranucci expressed shock over Lavitola’s alleged involvement, describing him as “a true friend” with whom he had almost daily contact.

Investigators are yet to establish a motive for the attack, but Lavitola is alleged to have instructed someone to identify people to plant the bomb, allegedly paying them several thousand euros.

The pair became friends in 2019 after Ranucci conducted a journalistic investigation into Lavitola, who in the past has been embroiled in several legal proceedings, including one resulting in a conviction for extorting the late former prime minister Silvio Berlusconi.

He said he had “never hidden” his friendship with Lavitola, today a businessman who owns a restaurant in Rome where Ranucci regularly eats, and that their last contact was when police were searching his home.

“He was agitated,” Ranucci told Corriere della Sera. “I was also very surprised by this turn of events in the investigation.”

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After the bomb attack, Ranucci said he had received so many threats that it would be difficult to trace who was behind it but that what happened was “a worrying new level” because it occurred “right in front of my home”.

Bullets were also found outside his home in 2024.

Ranucci’s investigations for Report focus on alleged crime and corruption. The show often targets government ministers, some of whom in turn have sued Report, and Ranucci has been a high-profile critic of alleged political interference in Rai by Giorgia Meloni’s far-right administration.

Ranucci has also written a book about the Italian mafia and in 2021 described how a former prisoner told him that mobsters “had given the order to kill” him after the book was published, but the hit “was stopped”.