Prosecutors say three people suspected of supporting the Islamic State group and thought to have been planning an attack in Europe were arrested in Switzerland earlier this year.

Swiss federal prosecutors said in a statement Friday that the Iraqi citizens, whom they didn't identify, were arrested in late March and have been held in pre-trial detention since.

They said the arrests weren't previously confirmed "so as not to jeopardize the outcome of enquiries still ongoing in Switzerland and abroad" to identify possible accomplices and support networks.

Prosecutors say suspected offenses by the three include misuse of explosives and toxic gases with criminal intent and encouraging illegal entry to, exit from and residence in Switzerland. They say they've been working closely with U.S. authorities and requested assistance from various European countries.