Detectives are appealing for two men seen in the company of Brendan McKernan last month in Belfast to contact them.
Brendan McKernan, 31, died in hospital on February 23. He had collapsed five days earlier.
Detective Inspector McCambridge said: "We know Brendan was in the company of two men at a fast food restaurant on Donegall Place at around 6.15pm on 18 February. We are keen to identify and locate these males.
"At this time Brendan’s death is not being treated as suspicious and we are seeking to speak with the two individuals as we work to establish the circumstances leading up to his death.
"We are also keen to locate a phone known to have been in the possession of the deceased but which was not found with him.
"The Nokia touchscreen device may contain evidence valuable to police but also contains precious photographs that the family wish to have returned."
Detectives can be contacted at Musgrave Reactive and Organised Crime by calling 101.