The Independent Police Complaints Commission (IPCC) is investigating the death of a man from Buckinghamshire.
Marcus Cottoy, 31, of High Wycombe, was arrested on suspicion of criminal damage on Saturday 21 March 2009.
He was taken to Amersham Police Station at 0045 GMT on the Sunday. He was later sectioned under the Mental Health Act.
He was being assessed at an NHS mental health unit when his condition "deteriorated". He was taken to Stoke Mandeville Hospital where he died.
The IPCC said it was investigating the case.
'No evidence of assault'
A post mortem examination was carried out by a Home Office pathologist which revealed Mr Cottoy had three packages in his bowel, which contained white powder.
One of these had ruptured. There was no evidence of an assault.
Thames Valley Police referred the case to the IPCC and an independent investigation is taking place.
IPCC investigators are checking CCTV, interviewing witnesses, including custody staff, and examining custody records.
IPCC Commissioner Mike Franklin said: "The death of a young man in these kind of circumstances can raise a range of concerns and it is important that Marcus's family and the local community can feel confident that an independent investigation will take place.
"To that end, we will endeavour to keep the public up to date with the progress of our investigation and make our findings public as soon as possible."
An inquest was opened and adjourned on Monday 30 March 2009.