Robert Hart, 26, died four days after the assault at Manchester's Parklife Festival in June 2014.
Greater Manchester Police made a fresh appeal for videos and photographs from the event at Heaton Park.
Mr Hart was attacked after an altercation, prompted by his girlfriend being hit with an inflatable doll.
Speaking ahead of this year's event, which takes place over the weekend, Det Supt Phil Reade, said: "We are one shot away from identifying Robert's attacker.
"We can only do this with the public's help."
He urged people to think back to the time and look again at any photos taken at the event.
"Were you there? Do you still have photographs or video footage from the festival?," he asked.
"Don't assume that someone else has sent the same or similar photograph or footage to what you have taken. The people we are looking for could be in the background of a photograph or video you have taken."
An inquest in 2015 concluded Mr Hart was unlawfully killed and died of a head injury following an assault.
The killer of a man who died after being punched at a music festival is "one shot away" from capture, police have said.