Officers from Greater Manchester Police would like Essex residents to help them identify the person who seriously assaulted a man at Parklife, who then later died.
It is thought the suspect could have come from any part of the country.
At around 9.15pm on Saturday, June 7, 26-year-old Robert Hart and his girlfriend were in front of the festival's main stage when he was attacked by an unknown man.
Robert was punched to the ground unconscious and was given CPR at the scene by two off-duty medical personnel.
He died in hospital on Thursday, June 12.
The offender is described as a mixed race male, aged in his late 20s and around 6ft 1 tall. He has a muscular build and short, dark hair which was shaved at the sides. He was also wearing shorts and a blue, bomber-style jacket with the letter 'A' in white on the front.
Police have also issued an efit.
Detective Inspector Richard Eales, of Greater Manchester Police, said: "Our immediate thoughts go out to Robert's family and loved ones. We currently have specially trained officers with them to help through this terrible time.
"We are still in the early stages of this investigation, however several witnesses have assisted us with vital information concerning the description of the offender. We believe a confrontation took place following an inflatable doll hitting Robert's girlfriend.
"We are aware other assaults took place in the park that evening, but I am specifically appealing for witnesses who recall seeing the inflatable doll and the attack on Robert immediately thereafter.
"I am keen for those witnesses to come forward and other persons nearby who may have mobile phone footage of the attack or captured an image of the person responsible.
"Finally, if you are the man responsible or were with him that evening, please come forward."
Anyone with information can contact the incident room direct on 0161 85 69283 or email the incident room on firstname.lastname@example.org any relevant footage.
Alternatively you can call police on 101 or independent charity Crimestoppers on 0800 555 111.