A Slovak reporter was investigating alleged ties between the country’s prime minister’s office and an Italian mafia network when he was killed this week, his colleagues and friends said Tuesday, as Europe’s latest murder of an organized crime reporter roiled this small nation.

Jan Kuciak, a 27-year-old investigative journalist, was set to publish a report alleging that Italy’s ’Ndrangheta mafia had fraudently received European Union funds designated for Slovakia before he and his fiancée were found dead Sunday, the friends and colleagues said. The ’Ndrangheta, based in the southern Italian region of Calabria, is one of the world’s wealthiest and best-connected crime groups, with branches throughout Europe.

Mr. Kuciak’s reporting touched on the group’s alleged ties to Slovakia’s head of government, Prime Minister Robert Fico. The prime minister’s principal state adviser formed a company with a man wanted in Italy for transporting goods for the ’Ndrangheta, according to Mr. Kuciak’s reporting published Tuesday in the Slovak news outlets Aktuality, for which he worked, and SME.

Police said Monday the killings appeared to be connected to Mr. Kuciak’s work. He was found shot in the chest and his fiancée shot in the head in the home they shared east of the capital, Bratislava.