A bus and part of the interchange in Frederick Street in the town centre went up in flames on Sunday night.
Detective Chief Inspector Sarah Poolman, of South Yorkshire Police, said: “While we’re in the early stages of this investigation, at this time we do believe the fire is suspicious and as such we will be making further enquiries in the community in the coming days.
“There was some disruption yesterday given the part-closure of the interchange building, so I am grateful for the public’s patience and understanding while we and our colleagues from South Yorkshire Fire and Rescue piece together the circumstances surrounding the fire.
“We’re keen to hear from anyone who was at the interchange on Sunday at around the time of the fire, did you see anything suspicious?”
Call South Yorkshire Police on 101.