Investigators from the Independent Police Complaints Commission want to hear from anyone who witnessed an incident on Christmas Eve after which a man was arrested and died while custody.
Investigators would like to speak to anyone who saw the man, from Bury St Edmunds, interacting with store staff before police arrival and his subsequent arrest outside Tesco, Fordham Road, Newmarket, at about 6pm on Christmas Eve.
Anyone with information is asked to contact the IPCC via Suffolk Police on 101, quoting CAD number 309 of December 24.
The IPCC had to be informed following his death.
It a statement, the commission said the 29-year-old man became unwell shortly after arriving at Bury St Edmunds Police Investigation Centre at about 6.45pm on Thursday. He was transferred to West Suffolk Hospital at about 7.20pm and died at about 9.40pm.
See the IPCC statement at the IPCC website