BRATISLAVA, Slovakia — A few months before his murder, Jan Kuciak received a disturbing phone call from one of Slovakia’s most prominent businessmen. Mr. Kuciak had written more than a dozen articles about the businessman, Marian Kocner, outlining a pattern of corrupt dealings made possible by his connections with the police and prosecutors. Mr. Kocner, long thought to have ties to organized crime, was furious.

“You can be sure that I will start paying special attention to you personally, Mr. Kuciak,” Mr. Kocner told the 27-year-old investigative reporter in September 2017.

Mr. Kuciak reported the threat to the police but, after more than a month, he wrote on Facebook that officers had yet to pick up the case. When they did, they dismissed it without even interviewing Mr. Kocner, according to the authorities.

In February 2018, Mr. Kuciak, 27, was shot dead in his home. His 27-year-old fiancée, Martina Kusnirova, was also killed there.