Swiss prosecutors said that the country’s officials had, in March, arrested three Iraqi citizens suspected of plotting an attack in Europe and of being supporters of the Islamic State group. Authorities said that the arrests were not made public earlier because they did not want to endanger the investigation.

Swiss officials are still conducting an investigation into the planned attack, the individuals and their accomplices inside the country, and their possible networks abroad. Prosecutors said that all three Iraqi nationals have been held in pre-trial detention since the end of March and requested assistance from other countries, including the U.S., in conducting the probe. The Office of the Attorney General of Switzerland, or OAG, also said that it has deployed a joint investigation team of officials from the Swiss Federal Criminal Police and the FBI.

“The suspected offences include supporting the Islamic State (IS), which is a criminal organisation, misuse of explosives and toxic gases with criminal intent, acts preparatory to criminal offences, pornography, and encouraging illegal entry to, exit from and residence in Switzerland,” the OAG said in a statement, adding that it is also conducting 20 more proceedings related to “militant Jihadism,” including five related to Syria.

The first tip-off was received by Swiss authorities through the country’s intelligence services, following which a criminal case was initiated against the three, a local news agency reported. The Islamic State group was banned by the Swiss government earlier this month.