Switzerland's top prosecutor has filed charges accusing four Iraqis linked to the radical Islamic State group of plotting terror attacks in Europe.

Attorney General Michael Lauber's office said Friday that three suspects, who were not identified, helped smuggle other IS followers to Europe, spread propaganda and provided unspecified "operative advice." They were detained in northeastern Switzerland in March and April 2014 and remain in custody.

The case was expanded on July 17 to include a fourth suspect who allegedly traveled to Syria to deliver radio equipment to Islamic State. Lauber's office said that suspect is accused of deleting information on Facebook relevant to the case involving a "high-ranking member of the criminal organization."

It said Swiss authorities have "particularly close" cooperation with the U.S. Justice Department over the case.