"The investigator has, within the legal timeframe (48 hours) at his disposal, checked the facts necessary to bring charges. After 48 hours, detainees were released," police said in a statement on Saturday.
The seven men were taken into custody on Thursday in police raids in the eastern town of Michalovice. Slovak media reported that among those held was businessman Antonino Vadala -- the owner of several companies -- and some of his relatives, alleged by Kuciak to have links to Italy's notorious 'Ndrangheta crime syndicate and contacts in the Slovak government.
In Italy, prosecutors have suggested that the notorious Calabrian crime syndicate the 'Ndrangheta may have been behind the killing of Kuciak, 27, and his fiancee Martina Kusnirova who were found shot dead at their home on Sunday.
The murder has raised concerns about media freedom and corruption in Slovakia and sparked mass protests at home and sharp condemnation abroad.
Thousands flocked to candlelit anti-corruption protests and memorials held across Slovakia on Friday. Organisers estimated that around 25,000 people gathered in the capital Bratislava, while thousands met in a dozen other cities and towns across the EU and Nato country of 5.4 million people.
Thousands of mostly young Slovaks joined anti-graft rallies last year demanding the dismissal of senior government and police officials for alleged foot-dragging on fighting graft.
Kuciak's fiancee was laid to rest in her wedding gown on Friday, as hundreds of mourners flocked to her funeral in the northern Slovak town of Gregorovce. The couple had been due to wed in May.
Kuciak's funeral will be held on Saturday in the northern village of Stiavnik, near the Czech border.