Swiss officials say police are conducting a raid in a mosque in the northern city of Winterthur.
Officials say that the police operation at An Nur mosque was continuing Wednesday morning, and that a regional prosecutor based in nearby Zurich would make a statement shortly.
Swiss daily Tages Anzeiger reported that investigators were focusing on the mosque's new preacher, who is from Somalia.
The newspaper wrote that in the past, a number of young men who were regulars at the mosque later left to join the Daesh terrorist group in Syria and Iraq.
In the wake of terrorist attacks in Europe in recent years, Swiss authorities have been stepping up controls and surveillance of suspected militants who could cause violence.