Mr Rigg was arrested after allegedly assaulting a police officer in Brixton
The sister of a man who died after his arrest in south London has asked for help in finding out exactly what happened to the 40-year-old.
Sean Rigg was taken to Brixton police station at 1915 BST on 21 August and was seen by a doctor when he fell ill.
His condition worsened and he was pronounced dead in hospital.
His sister, Samantha Rigg-Davies, hoped witnesses could explain more about the circumstances leading up to his arrest or how he seemed at the police station.
Mr Rigg had been arrested on suspicion of assaulting a police officer and for alleged public order offences.
The family were "deeply saddened by the sudden and untimely death of our brother", Ms Rigg-Davies said.
She appealed for anyone who may have seen Mr Rigg "walking around the Atkins Road area in Balham on the evening of Thursday, 21 August, witnessed the arrest, or were at the police station and saw Sean there, to come forward".
A preliminary post-mortem examination by a Home Office pathologist failed to show any obvious sign of injuries which could have caused the death of Mr Rigg, who lived on Fairmount Road.
On Saturday Ms Rigg-Davies said her brother had a mental illness but "was very fit".
"From the time of arrest to the time being brought into police custody he's collapsed and died. We've not been told why," she added.
The death has been referred to the Independent Police Complaints Commission, as is routine in such cases.
Investigators have obtained footage from security cameras operated by the Metropolitan Police and Lambeth Council.