The main suspect - who prosecutors believe has ordered the murder - is an entrepreneur with good contacts in politics and has been a key figure in Kuciak's investigation of corruption and slanderous politics.
27-year-olds Jan Kuciak and Martina Kusnirova were shot to death outside their home in Bratislava in February 2018.
The murders shook the country and triggered massive popular protests and eventually forced Prime Minister Robert Fico to resign.