A former soldier charged with killing investigative journalist Jan Kuciak has confessed to his murder, three sources including the akutality.sk news website where the victim had worked, reported on Thursday quoting police sources.

Kuciak and his fiance Martina Kusnirova were gunned down gangland style in their home in February 2018 as he was about to publish a report on alleged ties between Slovak politicians and the Italian mafia and associated irregularities in EU farm subsidy payments.

Aktuality.sk, owned by German publisher Axel Springer, quoted an unnamed "well-informed source" on Thursday saying that Miroslav Marcek, a former professional soldier had confessed to the double murder.

Two other Slovak media outlets, including public broadcaster RTVS and Dennik N, also reported that Marcek had confessed, quoting anonymous police sources.

Marian Kocner, a property developer with alleged links to the governing populist Smer-SD, was charged on March 14 with ordering the hit on Kuciak who had probed some of his suspect business dealings.

Marcek, and three other suspects have also been charged with the double murder and have been in police custody since October.

The double murder and Kuciak's explosive report plunged the eurozone country of 5.4 million people into crisis, triggered mass protests that toppled then Smer-SD prime minister Robert Fico in March 2018.

Sustained public outrage over the crime propelled Zuzana Caputova, an anti-graft environmental lawyer, to the presidency in March elections.

Kuciak had been investigating Kocner's business activities at the time of his death, one of several stories he had been working on for aktuality.sk.

Transparency International ranks Slovakia in 57th place in its list of the world's most corrupt countries and high-level corruption has long been a top election issue.