Three journalists from Finland’s largest daily are expected to appear in a Helsinki court on Thursday, suspected of publishing classified defence intelligence in an unprecedented case for the Nordic country renowned for its press freedom.

Two journalists at Helsingin Sanomat, and their former editor, who all deny any wrongdoing, may face a prison sentence of between four months and four years if found guilty of revealing national defence secrets in a report published in 2017.

The case is unusual in Finland because it has for years been among the top countries in a global press freedom ranking published annually by Reporters Without Borders.

But it slipped to fifth position this year, partly due to the upcoming court case, the journalist association’s Finnish branch said.

The 2017 investigative report by Helsingin Sanomat, entitled “Finland’s most secret place”, revealed the rough location and tasks of an intelligence unit of the defence forces at a time when Parliament was debating whether to expand its powers to monitor private data in digital networks.

According to the prosecutor, the article contained harmful information the publication of which was against the law.

Helsingin Sanomat Editor-in-Chief Kaius Niemi, who had charges against him in connection with the case dropped earlier for lack of evidence, said the journalists had not broken the law.

“We can show for each published sentence that the information could be found on the internet or in books prior to the publication of our article. Public information cannot be classified,” he told Reuters.

Hanne Aho, chairwoman of the Finnish Journalists’ Union, said the case was first of a kind.

“It is completely exceptional that Finnish journalists are being accused of high treason,” she told Reuters.

Aho said it was problematic that most of the legal proceedings in the case had taken place behind closed doors and called for the court to publish its reasoning to explain on what grounds freedom of speech could be restricted if the journalists were found guilty.

“The threat of a prison sentence can lead to self-censorship,” she said.

The trial will begin with a preparatory session at the Helsinki district court on Thursday.