Slovak Police commander Tibor Gaspar, has said the motive behind his murder was "most likely" related to Kuciak's investigative journalism.
The murder has raised concerns about media freedom and political graft in Slovakia, sparking mass protests at home and sharp condemnation overseas.
Police made seven arrests, including a businessman alleged to have links to the mafia in Italy and to officials close to Robert Fico, the Slovakian prime minister. "After the 48 hours passed, the persons were released from detention", police spokeswoman Denisa Baloghova said in a statement, as quoted by Reuters.
Mr Kuciak was investigating alleged ties between businessmen and mafia bosses in Calabria, southern Italy, who are said to have set up companies in Slovakia to benefit from European Union subsidies destined... "An attack on a journalist is also an attack on the freedom of our country, we must not allow it".
Thousands marched in Bratislava and other Slovak cities on Friday, demanding swift action in response to a killing that has shocked the country and shaken its government.
The latest protests echoed a wave of anti-graft rallies by mostly young Slovaks past year demanding the dismissal of senior police officials and ministers in Fico's government for alleged foot-dragging on fighting corruption.
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The killings come less than five months after anti-corruption journalist, Daphne Caruana Galizia, died in a auto bomb on Malta.
The seven Italians were taken into custody on Thursday in police raids in the eastern town of Michalovce. His last, unfinished article was published posthumously by Slovak and global media. But his government has offered a 1 million euro ($A1.6 million) reward for information aiding the investigation into Kuciak's murder.
Local media reports say he may have been killed because he had been investigating suspected collusion between the Slovak administration and the Italian Mafia. Both have denied any wrongdoing.
Kuciak alleged that Troskova, 30, had ties to Vadala, purportedly involved with 'Ndrangheta.
Christophe Deloire, the director-general of Reporters Without Borders, on Friday said he asked Fico to "express regret after insulting journalists and commit to avoid weakening investigative journalism".
But contacted by AFP, Harlem Desir, the Freedom of the Media representative at the Organisation for Security and Co-operation in Europe, confirmed that Deloire had called on Fico to apologise to journalists.