Police were called to the Marsh Farm area of Luton at around 2pm on Monday, November 4, after members of the public reported concerns about the behaviour of Leon Briggs, 39.
He was detained under section 136 of the Mental Health Act and taken to a Luton police station, where he became ill.
Briggs was then transferred to the Luton and Dunstable Hospital where he was subsequently pronounced dead.
IPCC investigators are carrying out house-to-house enquiries in the Marsh Road area and are appealing for any witnesses who may have seen the man’s interactions with police before he was taken into custody or at Luton police station to contact them.
IPCC Commissioner Mary Cunneen said: "Following a review of the evidence gathered so far, I have determined that this will be a criminal investigation.
"We are still in the early stages of this investigation and it is important that we do not pre-judge the findings. However, at this stage we believe there is an indication that potential criminal offences may have been committed including gross negligence and/or unlawful act manslaughter, misconduct in public office, and/or offences under the Health and Safety at Work Act 1974.
"We will also be considering whether any potential disciplinary offences have been committed."
She added: "We have made good progress in the investigation since Monday evening but we are still appealing for further witnesses.
"IPCC investigators have obtained and reviewed CCTV from the custody suite at Luton police station and at the junction of Marsh Road and Willow Way.
"House-to-house inquiries have been under way since Tuesday morning and I am extremely grateful for the response from the community. A number of independent witnesses have been identified, accounts have been taken and IPCC investigators will continue to interview witnesses over the coming days. Local businesses have also provided CCTV footage to assist our investigation."
Speaking at a vigil earlier this week, Leon Briggs' tearful brother Alan said: "We need everybody’s support in the community…wherever you are from, we do need justice for Leon.
"We won’t stop until we get justice. I’ll be here all day. Keep the flowers coming, keep the love. Keep the support, we need everybody."
The IPCC chief confirmed the police watchdog has been in touch with Briggs' family.
"But we still believe there are a number of other people who may have seen Leon on Monday and we are keen to speak to them, particularly if you were in the Marsh Road area or outside Luton police station", Cunneen said.
"We are in contact with Leon’s family and continue to update them regularly with the progress of our investigation. I know this must be an incredibly difficult time and my thoughts are with them.
"I want to stress again that this will be an independent and rigorous investigation into the circumstances of Leon’s death."
Another Black Man has died in Custody. What excuse will be given this time?