A post-mortem examination of a man who collapsed in police custody was inconclusive, the police watchdog said.
Leroy Junior Medford, 43, who died a day after being arrested in Reading, was named by the Independent Police Complaints Commission (IPCC).
The IPCC said it was awaiting the outcome of further tests, which will be passed to the coroner.
Thames Valley Police said it was "co-operating fully" with the investigation.
The force was obliged to pass the case to the IPCC because Mr Medford, who died on 2 April in hospital, first fell ill at Loddon Valley police station.
He was taken to the Royal Berkshire Hospital for a check-up following his arrest on suspicion of assault and a drugs offence, after officers expressed concern for his welfare.
Mr Medford was discharged the same day but was then found unresponsive in his cell, Thames Valley Police said.
The watchdog said no officers or custody staff were under investigation following his death, but that this would be "kept under review as the investigation progresses".
Guido Liguori, IPCC associate commissioner, said the investigation was examining the care given to Mr Medford while he was in custody and whether all risk assessments and relevant procedures were followed.