A man accused by Slovak prosecutors of having ordered the murder of investigative journalist Jan Kuciak used to be a director of a company registered in Malta.

The Slovak authorities said on Thursday they have charged entrepreneur Marian Kocner with having ordered the murder, which led to mass protests that toppled the prime minister. 

Mr Kocner was a director of International Investment Holdings, which was set up in 2010 but was struck off by the Company Registry in 2017. It was put in voluntary liquidation by the shareholders in 2016.

Four other people have already been charged with the February 2018 murder of Kuciak, who had been investigating Kocner's business activities in addition to alleged ties between Slovak politicians and the Italian mafia. 

A picture taken in Bratislava on February 20, 2019 shows candles beside portraits of Slovakian journalist Jan Kuciak and his fiancee Martina Kusnirova who were found shot dead at the couple's home near Bratislava.

Kuciak had been investigating Kocner's business activities at the time of his death, one of several stories he had been working on when he was gunned down with his fiancee Martina Kusnirova at home in February 2018.

"The reason for the murder was the journalistic work of the victim," a special prosecutor told reporters in Bratislava who was left unnamed for security reasons.

The prosecutor said Kocner, who has a number of property development and investment businesses, was charged on March 8 with "ordering the murder".

Kuciak's last explosive report, published post-humously and unrelated to Kocner, alleged ties between Slovak politicians and the Italian mafia.

The murders and the journalist's investigation plunged the EU member of 5.4 million people into crisis, raising concerns about media freedom and corruption.

Then prime minister Robert Fico was forced to resign but remains the leader of the ruling populist-left Smer-SD party and is a close ally of current premier Peter Pellegrini.

The latest move in the case comes days before the eurozone member's presidential election.

Last year, prosecutors charged four suspects in connection with the murders, including a woman identified as Alena Zs.

Local media reported that Alena Zs had worked as an interpreter for Kocner, who is believed to have ties to members of Smer-SD.

Peter Bardy, Kuciak's editor-in-chief at the aktuality.sk news website, claimed in September 2017 that Kocner had called Kuciak to threaten him.

Kocner had vowed to set up a website publishing information on reporters' private lives, according to the leading SME daily.

The 55-year-old multimillionaire, who owns more than a dozen companies, was detained by police in June on suspicion of fraud. He is still in custody.

"It is not excluded that further individuals will be accused of participating in the murder," the special prosecutor said Thursday.

The prosecutor added that "we do not yet have the weapon but we have evidence as to what kind of weapon it was".

Slovakia ranked 27 out of 180 countries in the Reporters Without Borders (RSF) World Press Freedom Index 2018, falling 10 places in a year.