Up to 20 teenagers were involved in kicking and punching to death a father of one in west Belfast, a senior detective has said.
Christopher Meli, 20, was beaten around the head in grassland in the Twinbrook area at the weekend.
Five male teenagers aged between 16 to 18 have been arrested in connection with the murder.
Police Service of Northern Ireland (PSNI) Detective Chief Inspector Richard Campbell said Mr Meli was the victim of a "sustained and vicious assault".
Mr Campbell said he believed a group of between 15-20 youths, some of them female, were involved in the attack.
He warned anyone who was there to come forward before detectives knocked on their doors.
"A number of witnesses have come forward and a picture is starting to emerge of what happened late on Friday night, running into early Saturday morning," he said.
"At this stage I believe that Christopher was one of a group of four people when he was set upon by a number of individuals from a much larger group.
"There appears to have been an initial incident on the Stewartstown Road before the attack on Christopher in an area known locally as Doc's Lane.
"Christopher became separated from his friends and he was fatally assaulted at the location where his body was found, in Doc's Lane.
"Some of those people who were in that larger group may not have been involved in the attack on Christopher, nor may they have anticipated the outcome of the attack.
"Police are working to establish exactly what happened and who was involved.
"It is only a matter of time before detectives identify everyone who was there and I am appealing for those people to come forward now and co-operate with this investigation by telling police what they saw. Come to us before we come to you.
"Anyone who was involved in the attack on Christopher should understand that they will be identified and it is in their interests to come forward now, voluntarily and with a solicitor if necessary, so that their role can be properly established."
The five teenagers in custody - two aged 18, two 16 and one 17 - are also being questioned about the attempted murder of another young man who was seriously assaulted in the same incident.
Mr Campbell said: "Christopher was subjected to a sustained and vicious assault, resulting in fatal injuries.
"His death has caused devastation to his family and shocked the wider community."