Witnesses are being asked to come forward as part of an investigation by the IOPC.
The man died just after midnight after being detained under the Mental Health Act earlier in the day.
Police have appealed for information after the girl went missing from Tibenham in south Norfolk.
Police were called after reports that a man was acting suspiciously, but after he was detained under the mental health act he became seriously unwell.
Police and an ambulance rushed to the scene, with an air ambulance landing in the park beside the Mere a short time later.
Police are still investigating the incident.
A local councillor has described the scattering of rodenticide around Ensign Way in Diss as "monstrously irresponsible"
The 51-second clip was in possession of a video showing a 12-month old baby being abused.
The man was sentenced to 100 months after armed robbery incidents in Diss, Hoxne and Fressingfield
The theft is one of a spate across the region and took place in in broad daylight in Park Road, Diss.
Equestrian items worth tens of thousands of pounds were stolen on Saturday