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.

Built in 1937, the concrete structure was the "last fragment" of the derelict seaside lido used for the street artist's "Bemusement Park" exhibit last year.
Conservationists said they had been told the fountain, in Weston-super-Mare, would be spared from destruction.
But North Somerset council said it was demolished due to its poor condition.
Visitors to Dismaland were greeted by the sight of the fountain upon entering, and it was picked out by a spotlight at night.
Its dilapidated appearance, however, was not the work of the elusive Banksy, but rather a truck which hit the fountain after the site became derelict.
The impact caused the upper cascade to tip over, leaving it sitting on its side atop the bottom part of the structure.
Weston Civic Society chairman Howard Smith said he felt "angry and upset" the fountain had gone.
"They [the council] gave us an assurance at the end of last year that it would be conserved," he said.
Mr Smith said the fountain survived the building of the Tropicana at a time when the original diving stage was destroyed.
"It was the last fragment of the actual bathing part of the original Art Deco pool.
"It was a beautiful object and it would not have taken much to erect it in another part of the town."
Work to turn the site into a space for arts and performances has been taking place since the end of 2015.
A spokesman for the council, which owns the site, said: "The top of the fountain was dislodged, the surround of the base was broken down and could not be used as a water reservoir, the main stem had numerous cracks in the structure and the edges on the lower main tier were damaged.
"Given its condition, the decision was made to take it down and demolish it."
Detectives investigating a fatal assault have released CCTV footage of potential witnesses who have yet to come forward.
[NEXT_CONCEPT]
The demolition of an Art Deco fountain which featured in Banksy's Dismaland show has been slammed by campaigners.