The country's police chief, Tibor Gaspar, told reporters Thursday that the individuals taken into custody were "persons mentioned" by Jan Kuciak in connection with the "Italian track".
The young reporter had been probing links between the Italian mafia and figures close to Prime Minister Robert Fico.
Among those held was businessman Antonino Vadala, the owner of several companies in Slovakia who is alleged by Kuciak to have links to Italy's notorious 'Ndrangheta crime syndicate and of being the former business partner of the prime minister's now former chief state adviser, Maria Troskova.
“Not everything is known," Ilya Lozovsky, an editor with the Organised Crime and Corruption Reporting Project (OCCRP), which had been supporting Kuciak's work, told RFI.
The OCCRP published the unfinished investigation on Wednesday, revealing how Italians with ties to the mafia had found a second home in Slovakia; Italians like Vadala.
"We’ve been upfront that this investigation was ongoing when it was so tragically cut short," says Lozovsky.
Kuciak and his fiancée, Martina Kusnirova, were found shot dead in their home on Sunday; the first killing of a journalist in Slovakia's history.
The fallout has forced the resignation of three government ministers.
Troskova and her former boss Viliam Jasan both resigned on Wednesday, saying they wished to prevent their names being linked to "these repellent crimes" and allow a full investigation to take place.
Slovakia's Culture Minister Marek Madaric, a member of Fico's Smer party, also resigned.
"Of course the most damning thing is that a reporter was killed while working on this, so clearly that indicates that it was in someone’s interests to put a stop to this," adds the OCCRP editor.
The reporter's research on alleged Italian mafia links with EU farm funds in Slovakia has been mentioned as a possible motive for his murder, which has raised serious concerns about media freedom and corruption in Slovakia and sparked international condemnation.
"I think this moment is very toxic, and it seems to me that Prime Minister Fico and the government is not reacting in adequate manner," Grigorij Meseznikov, president of the Institute of Public Affairs in Bratislava, told RFI.
Hundreds of people took to the streets of the capital on Wednesday in protest at alleged high-level corruption, carrying slogans such as "Mafia get out of my country".
"People are really angry that after years of democratisation and transition, mafia guys have still succeeded to have connections to prominent politicians," explains Meseznikov, suggesting that the prime minister may be living on borrowed time.
"I wouldn’t exclude that in the next few days, the opposition will succeed to vote a no-confidence vote to the prime minister in the parliament. Maybe the name of the prime minister of the Slovak Republic will be different in the next few weeks."
With opposition parties planning another mass protest for Friday 2 March, the government accuses the opposition of exploiting the murders for political gain.
Prime Minister Fico has been vehement in denying any links between him or his staff and the mafia. Bizarrely, he showed up at a press conference earlier in the week with a million euros in cash for anyone with information on Kuciak's murder.
The reporter's death adds to the toll of journalists killed in Europe over the past several years.
"Press in decline"
"We are all shocked that this takes place in the European Union," Mogens Blicher Bjerregard, president of the European Federation of Journalists in Brussels, told RFI.
"We had the bomb attack in Malta, Daphne Galizia, and now this shutdown of Jan Kuciak in Slovakia..."
Bjerregard decries a hostile environment for journalists, not just in Slovakia but also in neighbouring countries like the Czech Republic and Hungary, where media laws are becoming ever stricter.
“We have seen politicians being hostile to journalists and this is not creating a fruitful environment, so we’ve seen this hostile environment growing," he says.
"We have also seen how dangerous it has been for investigative journalists during the last months, years. In the Czech Republic now investigative journalists are now more and more under police protection."
To safeguard the work of journalists, Bjerregard is calling for closer cooperation between EU countries "where we organise ourselves better and help each other in this” and for Slovakia to finally adopt the European Council's recommendations for the protection of journalists.
As for Kuciak's unfinished story, the Organised Crime and Corruption Reporting Project and its partners have vowed to carry on reporting on what he never finished.
"Killing a journalist does not kill the story," says Ilya Lozovsky.
"Jan's death is a human tragedy but it's also an attack on our profession, and we're more committed than ever to getting the truth out no matter what it takes."