Two Slovak officials close to Prime Minister Robert Fico who were said to have ties with organized crime in a story by slain Slovak investigative journalist have resigned.
Fico has also offered a €1 million reward for information, but some journalists working at Aktuality.sk, a joint venture owned by German media group Axel Springer and Swiss group Ringier, have sought police protection because they still fear for their safety.
Kuciak's Aktuality.sk website reported Wednesday that he had been investigating activities by the Italian mafia in Slovakia and possible links to people close to Fico.
"Raids and detentions are being carried out in several places", Tibor Gaspar, Slovakia's chief of police, told reporters in the town of Michalovce, where a raid took place.
Police conducted raids in eastern Slovakia on houses linked to alleged members of the Italian mafia, about whom an investigative journalist was writing before he was shot dead.
Europol also said the 'Ndrangheta, which has its origins in Calabria in south Italy, ran a €2 billion money laundering scheme involving internet gambling firms in Malta.
Opposition groups have called for fresh anti-corruption protests in the capital Bratislava on Wednesday. Other anti-corruption protests are being planned for Friday across the country as well as in Prague, London and The Hague.
Fico fought back, accusing his opponents of using the deaths of two young people as "a simple political tool for the opposition, with which it wants to push people into the streets and gain power". "They started doing business, receiving subsidies, drawing European Union funds, but especially building relationships with influential people in politics - even in the government office of the Slovak Republic".
The killings, the first murder of a journalist in Slovakia, horrified and shocked the country.
Jasan and Troskova denied any links to the murder.
Kuciak's report was part of an worldwide effort involving the global Organized Crime and Corruption Reporting Project (OCCRP), the Czech Center for Investigative Journalism (CCIJ) and the Investigative Reporting Project Italy (IRPI).
He, like fellow murdered Maltese journalist Daphne Caruana Galizia, worked on the Panama Papers scandal.
"This murder will change the atmosphere in Slovakia in a way that there will be much harder pressure on the investigation of serious crimes, on revealing corruption and clientelism, on revealing interconnection of politics and business".
In a message posted on Facebook, Robert Kalinak says he has been in touch with "the most important investigative agencies in the world", having asked for help in the wake of the discovery of the bodies of Jan Kuciak and his girlfriend Martina Kusnirova at their home on Sunday.
According to earlier media reports, Maria Toroskova, a senior advisor to PM Fico, was among those being probed.