The probe’s announcement comes after local media leaked alleged text messages in which the suspects boasted of their ties to several top-level politicians including then prime minister Robert Fico.
He was forced to resign because of the fallout from the killing.
“Representatives of state authorities at various levels communicated with Marian” Kocner, the entrepreneur charged with ordering the murder, an unnamed special prosecutor told reporters, without providing details about the individuals.
“I urge these people to come forward and personally explain this communication at this stage,” he said, adding that prosecutors have reason to believe the officials committed crimes.
Journalist Jan Kuciak, 27, had been investigating Kocner’s business activities when he and his fiancee Martina Kusnirova were gunned down at home in February 2018.
Mass anti-government protests forced Fico to resign, but he remains the leader of the governing populist-left Smer-SD party and is a close ally of current Prime Minister Peter Pellegrini.
Prosecutors have charged five people over the double murder, including property developer Kocner and his one-time interpreter, named as Alena Zs.
Kocner maintains his innocence, according to his lawyer Marek Para, who did not refute the authenticity of the leaked text messages to reporters but said their contents make clear “that they don’t concern the act (of murder) at all.”
In his leaked chats with Alena Zs., which the police have examined, Kocner refers to Fico as “Squarehead”, a widely used nickname for the politician.
“I’m worried Squarehead won’t weather this,” Kocner allegedly wrote amid the public outcry after the murders came to light.
Weeks later, after Fico had already stepped down, Kocner informed Alena Zs. that he was “going to pay the Squarehead a surprise visit and kick him in the arse.”
Last week, Fico blamed the opposition and the media for linking his party to Kocner, telling reporters that their coverage against the party was “turning into a kind of jihad”.
Bratislava-based analyst Pavol Babos told AFP that “it cannot be ruled out that Kocner boasted about things that were not true in an attempt to make an impression.”
He said it was too early to tell whether the messages would influence voters in general elections set for March 2020.
But he added that “both Robert Fico and (ousted interior minister) Robert Kalinak are definitely losing coalition potential.
“I cannot imagine political parties entering into a coalition with Smer-SD.”