Slovakian authorities have charged a controversial businessman with ordering the double murder of an investigative journalist and his fiancee last year in an attack that shocked the country and led to a series of street protests.

At a press conference on Thursday afternoon, special prosecutors said a man named Marian K had been charged with the murder of Ján Kuciak and Martina Kušnírová, who were shot dead in their home last February.

Protests held in the aftermath of the killings called for an end to corruption and official impunity, which resulted in the long-serving prime minister, Robert Fico, stepping aside.

The prosecutors did not give the full name of the suspect, in accordance with Slovakian law, but the man in question was widely reported by Slovakian media to be Marián Kočner, a businessman who had previously threatened Kuciak.

Kočner, a 55-year-old multimillionaire, owns more than a dozen companies, and has been in custody since last June, when he was detained on suspicion of fraud. He is in custody in the central city of Banska Bystrica.

Four suspects were charged with carrying out the killing but the alleged mastermind had not been publicly identified until now.

“The reason for the murder was the journalistic work of the victim,” one of the special prosecutors told reporters in Bratislava. He said there might be further arrests in the case, and added that while the weapon had not been found, there was evidence of what kind of weapon had been used.

From the outset, police have worked on the assumption that it was a contract killing linked to Kuciak’s work. A data journalist with the online portal Aktuality.sk, Kuciak was investigating alleged government corruption and mafia links with Slovakian politicians. Both he and Kušnírová were 27 at the time of the killing.

“I was not surprised, because Marian Kočner was practically from the first days after the murder among a group of potential suspects,” wrote Peter Bárdy, the editor-in-chief of Aktuality, in a column posted on the publication’s website.

Bárdy said Kočner had previously threatened Kuciak, who complained in a Facebook post a few months before the killings that he had reported a threat to police but feared they were not acting on it.

Bárdy said it was for the courts to prove Kočner’s guilt, but that if he was indeed guilty, he should be punished harshly, having long operated outside the law. “Kočner had influential friends in politics, the police, and the prosecutors. They protected him.”

On the first anniversary of the killings last month, around 30,000 people marched in Bratislava and thousands of others gathered across the country. Although Fico stepped aside last year, he is still considered to be the most powerful politician in the country, and his Smer party is still in government.

To coincide with the anniversary, a group of media freedom organisations penned an open letter to Slovakian authorities demanding answers to three questions: whether the authorities knew or should have known about an imminent threat to Kuciak; whether steps were taken to protect him, and what steps would be taken to protect Slovakian journalists in future.

The news of the arrests comes two days before Slovaks vote in the first round of a presidential election. The front-runner, Zuzana Čaputová, was part of the protest movement against the government and is calling for a new and more inclusive type of politics. She is up against Maroš Šefčovič, a vice president of the European Commission who is running as an independent but is backed by Smer.