France's top security official said the government has expelled "about 10" radical imams and preachers since the beginning of the year and a total of 40 since 2012.
Interior minister Bernard Cazeneuve told Europe 1 radio that France will not tolerate "preachers of hatred".
He spoke three days after Yassin Salhi was accused of beheading his boss and then trying to cause an explosion at a US-owned gas factory in Saint-Quentin-Fallavier near Lyon, south-eastern France.
He had been flagged in 2006 for ties to radical Muslims, but surveillance was lifted in 2008.
The plant reopened on Monday. Executives say Salhi made regular deliveries for the plant and had an entry badge.
Authorities are searching for possible international links.