In a case that made headlines at the time, the 39-year-old man had vehemently resisted arrest on August 7, leading officers to pepper-spray him, reports dpa news agency.
Eight police officers are under investigation for grievous bodily harm to the man, who died shortly after his arrest in the western German town of Oer-Erkenschwick.
A post-mortem has not revealed what caused the 39-year-old to die, the public prosecutor’s office said on Sunday.
The man is thought to have been under the influence of drugs.
German police have faced numerous accusations of brutality during protests and arrests, with scrutiny raised in the wake of mass demonstrations in the country and elsewhere in response to widely shared footage of the killing of George Floyd during an arrest by US police.
According to police, around 150 onlookers followed the police operation at the time, many of them filming it with their cellphones.
According to the current status of the investigation, there are indications that in at least one case, officers could have deleted a film recording on a bystander’s cell phone, the public prosecutor’s office said.
This could be considered evidence of suspected coercion by officers.
Four officers are now being investigated for coercion.
The public prosecutor said that a specialist firm has been commissioned to restore possibly deleted videos on the bystanders’ cell phones.
At the same time, the investigators appealed to other witnesses of the operation to contact the police.