PEZINOK, Slovakia (Reuters) - Four suspects in the murder of a Slovak investigative journalist and his fiancée will go on trial in January after judges ruled on Thursday that there was enough evidence to proceed in a case that has triggered mass protests against corruption.

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Jan Kuciak and Martina Kusnirova, both 27, were gunned down in their house outside the capital Bratislava in February 2018.

Prosecutors say entrepreneur Marian Kocner, a subject of Kuciak’s reporting on fraud cases involving politically connected businessmen, had ordered his killing.

The murders stoked widespread public anger and forced Prime Minister Robert Fico to resign. His ruling Smer party faces a tight election on Feb. 29.

The case is a test of Slovak judicial independence given that the investigation exposed links between Kocner and police and public officials.

“We will be satisfied only when all the cases Jan wrote about are solved,” the mother of one of the victims, Zlatica Kusnirova, told reporters as she arrived at court. “Then their deaths will make at least some sense. It’s not enough to convict the murderers.”

At Thursday’s hearing at the Special Criminal Court in Pezinok near Bratislava, guards in balaclavas and bearing automatic rifles led the handcuffed defendants into the courtroom.

The prosecutor and all defendants - including two men who allegedly carried out the murder and a woman accused of being an intermediary - said no when asked by the judge if they wanted a deal for a reduced sentence in exchange for confession.

Alleged intermediary Alena Zsuzsova said: “I am not interested, I am innocent.”

All face up to life in prison if convicted.

The panel of three judges rejected objections from defense lawyers, who argued there were irregularities during the investigation and said the case should be returned for more evidence-gathering. The judges said the main trial would start on Jan. 13.

The court could impose a reduced sentence if any of the defendants confess during the trial. Slovak media and Kuciak’s family lawyer have said one of the accused admitted during the investigation to be the shooter.

A fifth man, who will be tried separately, has confessed to facilitating the killing and has agreed a plea deal with prosecutors to act as a witness.

The investigation has widened since prosecutors extracted tens of thousands of messages from Kocner’s phone containing communication with “representatives of state bodies and the justice system”.

The revelations caused several senior officials to resign. Slovakia’s former chief prosecutor was charged with abuse of power in a case also involving Kocner.