Ontario's Special Investigations Unit has been called in after an incident involving Thunder Bay Police on Wednesday afternoon in the Academy Heights area.

Police said they arrested a suspect after three men — one possibly carrying a gun — were reportedly chasing another man.

The suspect was transported to the Balmoral Police Station. The SIU said the man then lost all vital signs when he was in the police booking room. He was transported by ambulance to the Thunder Bay Regional Health Sciences Centre.

In the interim, officers from Uniform Patrol, Community Services and the Emergency Task Unit searched the Academy Heights area with assistance from the OPP K-9 unit. Two other men believed to be involved in this incident were located at 2:36 p.m. and arrested in an Academy Drive apartment. At the time of the arrest, a firearm and ammunition was located in the apartment. The investigation continues.

Details concerning the man in hospital have not been released, although he was reported to be in stable condition Wednesday night.

The province's Special Investigations Unit has invoked its mandate to investigate the circumstances surrounding the man who was in medical distress.