Kurt Pelda, was who was named Swiss journalist of the year in 2014, told local newspaper Der Landbote the terrorist group established the cell around the An'Nur mosque in Winterthur's Hegi neighbourhood.
“A man from Winterthur became radicalized in the last few months and recently travelled to Syria,” Pelda said.
“We do not know exactly what he has done in Syria but there is every indication it has to do with Isis.”
This follows reports of four other young people travelling from the city to Syria.
Pelda told Der Landbote he believes someone is masterminding the cell from the mosque, “let there be no doubt”. He added: “There is a whole bunch of preachers and prayer leaders who actively meet in the mosque or outside with young people and trying to radicalize them.”
However, Atef Sahoun, president of the An'Nur Islamic Cultural Association, denied such activity was occurring.
“If we discover radical tendencies in one member then the particular person will be immediately excluded,” he told Der Landbote.
“We send them away, no matter who it is.”
Pelda, however, said he plans to publish research on the Isis activity in the coming days.
And Blerim Bunkaju, a Muslim and socialist party politician in Winterthur, said he heard a few months ago of several young Muslims becoming radicalized in the city, the Tages Anzeiger newspaper reported on Tuesday.