Then, as now, there were even powerful oil interests in the affair — possible payments to Mussolini’s brother Arnaldo Mussolini, the Jared Kushner figure of the situation — that contributed to the consensus to “get over” the brutal killing.

Then too, the reaction of near-universal horror to the Matteotti killing was surprising since violence had been a consistent feature of Fascism. The thugs who killed Mr. Matteotti had already used violence, intimidation and fraud during the 1924 elections that gave Mussolini a majority in Parliament. The world — and most middle-class Italians — credited Mussolini with preventing a Bolshevik-style revolution in Italy and was prepared to overlook what it took to be a little residual violence that it assumed would fade away after Mussolini started wearing the bowler hat and spats of a respectable politician.

Italy was still a half-functioning democracy, and that helped expose the murder and the lies deployed during the attempts at a cover-up. Eyewitnesses had seen Mr. Matteotti being forced into a car. An elderly couple had seen the killers’ Lancia automobile as they staked out Mr. Matteotti’s house in the days before the kidnapping and wrote down the license plate.

The police found the car’s upholstery covered in Mr. Matteotti’s blood. He had been stabbed repeatedly and his body mutilated. A comparatively independent initial investigation linked the killers to Mussolini’s office, where one of his top aides, Cesare Rossi, directed his press office and ran Ceka Fascista, the private hit squad named after Cheka, the Bolshevik predecessor of the K.G.B.

Mr. Matteotti was killed before his parliamentary speech because he had been gathering evidence and planning to expose corruption in the assigning of a major oil concession by Mussolini’s highly personalized government to an American company, Sinclair Oil.

Mussolini got the case transferred to more cooperative investigators who concluded that the Matteotti killing had been unplanned and involuntary. The passage of time, the dithering of the Italian political class, the self-interest of foreign governments and the complicity of the international press helped Mussolini survive.

A remarkable new book, “La Scoperta dell’Italia,” by the Italian historian Mauro Canali, describes how the Fascists manipulated foreign journalists during the Matteotti crisis. At the time, two of the most important media outlets in Italy, The Associated Press and The New York Times, were locally headed by a father-son team, Salvatore and Arnaldo Cortesi, who were Italian citizens and ardent Fascist supporters.