Leon Briggs, 39, from Luton, died after being held following concerns about his behaviour in the town last year.
Five Bedfordshire police officers and a detention officer are being investigated by the Independent Police Complaints Commission (IPCC).
A second detention officer is no longer under investigation.
In November 2013, members of the public raised the alarm when they became worried about Mr Briggs's behaviour in Marsh Road, Luton.
He was detained under the Mental Health Act and taken to Luton police station. He became ill in custody and was taken to Luton and Dunstable Hospital, where he later died.
Seven members of Bedfordshire Police were suspended in November 2013 as an IPCC investigation began.
All were interviewed under criminal caution by IPCC investigators on suspicion of offences including gross negligence and/or unlawful act manslaughter, misconduct in public office, and/or offences under the Health and Safety at Work Act 1974.
Four of those police officers - a constable and three sergeants - have been served with additional gross misconduct notices, "relating to the matter of conferring at the police post-incident procedure on 4 November 2013, contrary to the College of Policing's Approved Professional Practice," an IPCC statement said.
Further criminal interviews have taken place with the five police officers and detention officer and statements from more than 150 witnesses who saw Mr Briggs on 4 November, have been gathered.
The second detention officer has been told they are "no longer under criminal or misconduct investigation" but has been interviewed as a "significant witness", the IPCC said.
Mr Briggs' family and Bedfordshire Police have been updated with the latest developments.
Four police officers currently under investigation over the death of a man in custody, have been served with additional gross misconduct notices.