The intermediary in the February 2018 double murder of a Slovak investigative journalist and his fiancee on Tuesday accused wealthy entrepreneur Marian Kocner of having ordered the hit that shook the nation.

Zoltan Andrusko, who was sentenced to 15 years in jail on a plea bargain last month, was testifying in court against his alleged co-conspirators in the shooting of reporter Jan Kuciak, who had been probing Kocner's business activities, and his fiancee Martina Kusnirova at their home near the capital Bratislava.

The double murder triggered mass protests that toppled then-premier Robert Fico -- who is however still the head of the Smer-SD party poised to win the February general election -- and has become a lightning rod for outrage against high-level corruption in the EU member state.

"I mediated the murder of Jan Kuciak between (suspected co-conspirators) Alena Zsuzsova and (Tomas) Szabo at the direction of Mr Kocner," Andrusko testified at the Special Criminal Court in the western city of Pezinok, 20 kilometres (12 miles) outside Bratislava.

"I'm so sorry about what happened. I don't know how to fix it, I guess I can't, but I'll do everything I can to set things right. With my testimony, of course," Andrusko told the parents of the murdered couple.

He claimed Zsuzsova, a friend of Kocner's, handed him 50,000 euros ($55,000) the morning after the killings to give to confessed gunman Miroslav Marcek and his cousin and alleged sidekick Tomas Szabo, who denies involvement.

Andrusko added that Zsuzsova, who on Monday denied arranging the hit, had been "furious" when he told her the fiancee had also been killed.

"She was angry... but she said okay, that she would go meet Mr Kocner and then we would talk. She then met me in the morning and gave me money."

The hit was also botched in another way: Andrusko testified that Zsuzsova had originally requested that "the boys were to take Jan Kuciak somewhere and assassinate him so that his body was not found."

According to Andrusko, when Marcek and Szabo asked who had ordered the hit, "we agreed with Alena that I should not mention Kocner in any way. Rather, I should say that there was a certain Russian person who has problems with the media in Slovakia."

Slovak businessman Marian Kocner has been accused of ordering the hit on an investigative journalist that shook Slovakia and toppled the prime minister Slovak businessman Marian Kocner has been accused of ordering the hit on an investigative journalist that shook Slovakia and toppled the prime minister Photo: AFP / VLADIMIR SIMICEK

On Monday, Kocner refused to comment on his murder indictment, citing "objections" to the contents.

According to the indictment, Kocner decided "to get rid of Jan Kuciak physically and thus prevent further disclosure of his (Kocner's) activities" after failing to find "any dirt" to discredit the journalist.

Kocner had previously pleaded not guilty to ordering the hit on Kuciak.

The parents of the murdered couple testified earlier Tuesday, with Kusnirova's mother Zlatica telling the court that "from the first moment I said it was a political murder".

After the mother asked "who gave you the right to take our children's lives?", Zsuzsova rejected the accusation and invoked the presumption of innocence.

Jozef Kuciak, the father of the late journalist, said he had once voted for the Smer-SD party.

"I regarded myself a Social Democrat and believed in all that gibberish. That is why I was not afraid of Mr. Kocner's threats, because I thought that nothing like this could happen in our country," he told the court.

"I thought we had a functioning prosecutor's office, police, courts. But Jan had warned me that I shouldn't believe everything politicians say," he added.

The protests triggered by the murder that brought down the government paved the way for the election of liberal anti-corruption activist Zuzana Caputova as president last March.