20 December 2015 Last updated at 13:02 GMT
Andrew Russell, 36, was found unconscious in a car park on Bradshaw Way, in Derby, shortly before 02:00 on 16 November. He was taken to hospital but later died.
Det Insp Graham Prince, of Derbyshire Police, said: "We are trying to trace a number of cars seen driving along London Road between 1.30am and 2.15am that morning, along with several lone people walking down the road during the same time period.
"These people have yet to come forward and they could have information which may help with the inquiry."
A 41-year-old man has been charged in connection with the alleged attack.
Detectives investigating a fatal assault have released CCTV footage of potential witnesses who have yet to come forward.