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, prosecutors said Friday.

The Iraqi citizens were arrested in late March and have been held in pre-trial detention since then, Swiss federal prosecutors said in a statement. They didn’t give the names or any other details of the suspects.

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, they said.

Prosecutors say suspected offenses by the three include misuse of explosives and toxic gases with criminal intent, “acts preparatory to criminal offenses,” an unspecified offense or offenses involving pornography and encouraging illegal entry to, exit from and residence in Switzerland.

Swiss authorities have requested assistance from various European countries over recent months and have been working closely with U.S. authorities, prosecutors said, adding that a joint investigation team has been set up with officers from the FBI and Switzerland’s federal criminal police.

Prosecutors gave no details on where, when or how the three might have been planning to carry out an attack.

(AP)