The murder of a Slovak investigative journalist and his fiancee last month, which sparked mass protests and forced the prime minister, Robert Fico, to resign, is likely to have been a contract killing, a prosecutor has said.

Ján Kuciak, 27, was shot twice in his chest and Martina Kusnirova, 27, was shot in the head at their home outside Bratislava, said the prosecutor, who asked not to be identified.

Police found the bodies on 25 February after relatives reported that the couple had not been answering phones for days.

No one has been charged with the murder and police and prosecutors have kept quiet on the investigation.

“There were no signs of struggle and most likely nothing had been stolen, which suggests it was a contract killing,” the prosecutor said. “Kuciak worked on a number of stories. The police are pursuing several lines of investigation with equal attention.”

Kuciak reported on Slovak businessmen mentioned in the Panama Papers and also looked into fraud cases involving businessmen with Slovak political ties. He was also looking into suspected mafia links of Italians with businesses in Slovakia.

In his final report, published posthumously, he said one of the Italians had past business links with two Slovaks who later worked in Fico’s office.

The Italian was briefly detained in the investigation along with six others. He was detained on 13 March and placed in custody in an unrelated case of suspected drug trafficking.

Both of Fico’s aides have resigned but deny connections to the murder. Their Italian former business partner has denied having connections with either the mafia or the murder.

The killings led to the biggest protests in the country since the end of communism in 1989, as tens of thousands of people demanded a government shake-up or new elections.

Fico resigned this month, having leding Slovakia for 10 of the past 12 years. His three-party coalition has been reformed under a new prime minister, Peter Pellegrini, a long-time senior member of Fico’s Smer party. The new cabinet has already faced public protests as well.

Protesters see the reshuffle as insufficient to safeguard a fair investigation of Kuciak’s murder. Smer was often a target of the reporter’s investigative journalism.