There were at least six them and they had assistance from some police officers. The group that monitored journalists before the murder of Ján Kuciak per order of Marian Kočner, consisted mainly of former secret service agents.

The Sme daily analysed documents from the investigation the editorial legally acquired. Sme daily reporter Adam Valček was one of the journalists under surveillance and thus authorized to look at the investigation file. The documents in the file show the distribution of tasks in the group, the costs of the surveillance, and where the information they gathered ended up.
When journalists asked then interior minister Robert Kaliňák back in 2017 what he thought of Kočner's statement that he would search for dirt on Kuciak, he downplayed the concerns. "That is what you do, search for dirt," he told journalists.

Kuciak filed a criminal complaint about it then, but the police rejected it. In February 2018, Kuciak and his fiancée Martina Kušnírová were murdered in their home in Veľká Mača. After the murder, Kaliňák said he would speak differently if asked again.
In mid-March 2019, the police charged Kočner with ordering their murder. The information about Kuciak, used by the people suspected of having committed the murder, came from the surveillance conducted by Kočner's people.
A friend from the secret service
It is now a known fact that Kočner assigned his friend Peter Tóth, a former journalist-turned-spy, to surveil journalists for him in 2017.
On March 26, 2019, the police initiated a criminal prosecution against an unknown perpetrator. Based on the investigation documents, unknown perpetrators have followed journalists and other persons from early 2017 to May 2018.
"They took pictures, videos, and wrote analyses of their activities," the document reads.