The victim died in hospital after he was shot just after 2am on Friday in the custody suite in Windmill Road.
The 23-year-old murder suspect, who police said turned the weapon on himself, remains in a critical condition in hospital.
It is understood that he was not regarded as a subject of interest by security services, but reports suggest he may have previously been referred to the anti-extremism Prevent programme.
Regional director Sal Naseem said the IOPO has established that the man was arrested for possession of class B drugs with intent to supply and possession of ammunition.
In a statement he said: “The man was handcuffed to the rear before being transported to Croydon Custody Suite in a police vehicle where he was escorted into the building. He remained handcuffed to the rear and seated in a holding area in the custody suite.
“His handcuffs remained in place while officers prepared to search him using a metal detector.
“It is at the point that shots were fired resulting in the fatal injuries to the officer and critical injuries to the man. A non-police issue firearm, which appears to be a revolver, has been recovered from the scene. Further ballistic work will be required.”
The IOPC investigates all cases where a member of the public dies or is seriously injured in police custody.
Mr Naseem added: “We began our investigation in the early hours of Friday September 25 and our investigators have been at Croydon Custody Centre in Windmill Road, south London, gathering evidence. Our investigators are also in the area where the man was detained gathering relevant CCTV evidence.
“We will be looking at all of the police contact with the man, from the time he was detained leading up to today’s tragic incident.
“It is understandable there will be speculation and questions asked about how this incident unfolded and we are seeking answers. It is important that we independently establish the sequence of events.
“Given the clear trauma suffered by those officers who were present during the incident, they have not yet provided their initial accounts. The work to obtain these will continue next week.
“We have obtained CCTV from the custody centre and body-worn video footage from the officers present. This will be reviewed and analysed over coming days.”