Kuciak's last story for Aktuality, on February 9, looked at transactions by firms linked to businessman Marian Kocner and connected to a Bratislava luxury apartment complex that became the center of a political scandal a year ago.
Police in Slovakia said Thursday that they have detained three businessmen named in a report on organized crime and corruption that was written by a slain Slovak journalist and published posthumously earlier this week.
He was echoed by Italy's anti-Mafia prosecutor, Gaetano Paci, who called the journalist's murder a "milestone".
The government of Slovakia said Monday it was offering a reward of 1 million euros ($1.2 million US) for information about the killing.
The murders have raised concerns about media freedoms and corruption in Slovakia and sparked a wave of worldwide condemnation. 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". President Andrej Kiska, who defeated Fico in 2014 to win election, is planning to take the unusual step of holding a televised address to the nation on Sunday.
Slovak national police chief Tibor Gaspar, however, disputed that claim.
"I can confirm that these raids and detentions are being carried out in several places", said police commander Tibor Gaspar, according to the French news agency AFP.
Troskova and the secretary of the country's Security Council, also mentioned in Kuciak's reporting, resigned on February 28 pending results of the investigation. The increasing pressure is a blow to a leader who has diverged from his peers in ex-communist eastern Europe by edging away from rising euroskeptic sentiment and trying to solidify Slovakia's place at the core of the European Union.
He, like fellow murdered Maltese journalist Daphne Caruana Galizia, worked on the Panama Papers scandal.
"There is a dirty mess all around here. and we common people have to put up with it", protester Jozef Belovic, 58, said as he walked through the capital in sub-zero temperatures along with an estimated 10,000 to 15,000 others.
Fico, who once told journalists they were "dirty, anti-Slovak prostitutes" and used terms like "plain, silly hyenas" and "slimy snakes" to describe the media, has vowed his government is committed to the "protection of freedom of speech and the safety of journalists".