One of four suspects in the 2018 murder of Slovak journalist Jan Kuciak and his fiancee admitted to the killing in court on Monday.

Miroslav Marcek, a 37-year-old former professional soldier, told the jury at the Special Criminal Court in the western city of Pezinok near Bratislava: "I am guilty ... I knocked on the door, Mr. Kuciak opened, I shot him in the chest."

Referring to Kuciak's partner, Martina Kusnirova, Marcek continued: "Unfortunately, I saw that there was one more person, she ran into the kitchen and [I] shot her there.

"Seeing them on TV and seeing their pain made me tell what happened ... I'm sorry, but I can't undo it."

More:

But the main suspect in the case, Slovak businessman Marian Kocner, pleaded not guilty to ordering the assassination of the journalist who had been probing his business activities.

Kuciak was an investigative reporter focused on tax fraud carried out by the country's wealthy and powerful - top business tycoons, often with links to senior political figures.

The killing - the first murder of a journalist since the country's independence in 1993 - led to large demonstrations against graft that forced Prime Minister Robert Fico and the country's police chief to step down.

Demonstrators take part in a protest rally marking the first anniversary of the murder of investigative reporter Jan Kuciak and his fiancee Martina Kusnirova in Bratislava, Slovakia on February 21, 2019 [File: David W Cerny/Reuters]

Last month, Zoltan Andrusko, one of the accused who was an intermediary in the murder plot, was sentenced to 15 years in jail after agreeing a plea bargain. He will be testifying at this trial.

Prosecutors argue that Andrusko served as a go-between in the murder, hiring gunmen Marcek and his cousin Tomas Szabo at the request of his friend Alena Zsuzsova, who was in turn following orders from mastermind Kocner.

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.

The protests paved the way for the election of liberal anti-corruption activist Zuzana Caputova as president last March.