The Centre on Tuesday sent in about 400 troops of the paramilitary Border Security Force or BSF to West Bengal after a request from the state government trying to control communal trouble near Kolkata. Tension flared in several places in North 24 Parganas district over an offensive Facebook post on Friday about Prophet Mohammed by a 17-year-old student. The student has been arrested, but there have been violent protests in his town Baduria, about 70 km from Kolkata, and other areas nearby.