A packed cinema had to be evacuated when a smoke grenade was set off during a film at the weekend.
Fire crews were called to clear smoke from the UCI Cinema in Fenton Street, Scunthorpe, at 2030 GMT on Friday.
A Humberside Police spokesman said: "It is believed youths sitting in the back row may have been responsible."
He added that during the evacuation, a 13-year-old boy was punched "for no reason" by a heavily-built man aged about 30 years, with blond hair.