Fifteen people required medical treatment after a substance was sprayed in branches of McDonald's, Pizza Hut and Tesco yesterday evening.
West Midlands Police have confirmed the substance was sprayed in three businesses in Upper Gornal, Dudley, shortly after 7pm.
Paramedics treated 15 people at the scene with breathing difficulties, one of whom was taken to hospital. None suffered long-lasting effects.
Police are tracking CCTV and speaking to witnesses while forensics experts work to establish the nature of the substance sprayed.
Anyone with information is asked to contact West Midlands Police.