A nightclub promoter shot in the head in south-east London a year ago was killed over a territorial drugs dispute involving his friends, police said.
Daniel Herbert, 27, of Orpington, was found with a bullet wound to the head in Bermondsey on 17 August 2009.
Det Insp Richard Beadle said the victim was "singled out by association" and killed as some form of "retribution".
Mr Herbert's mother appealed to his friends to break out of the "Bermondsey mentality" and speak to the police.
Police are searching for five men who were seen chasing Mr Herbert.
Mr Herbert was chased, beaten and then forced to kneel before his attackers shot him in the head at close range. His body was found in Clements Road.
Det Insp Beadle said: "There is a link to low-level drugs within this area. Daniel himself, in the lead up to his murder, has been on the periphery of this.
"There are other people who are directly involved, and I can categorically say he was singled out by association on that particular night.
"It's unfortunate he found himself alone and facing the wrath of this group of people for somebody else's actions."
Speaking at the scene of the murder, the victim's mother Androulla Herbert said: "It doesn't get any easier, Daniel was my only child, he was my every day. The days are empty and quiet.
"I'm sure a lot of his friends know something.
"But they haven't spoken to police. It's the Bermondsey mentality, do not speak to the police, even when your friend has been murdered. They are probably scared too.
"How can you cover up or know your friend has been involved in a murder and not say anything? That is why I find it really hard. It's killing me."
Police have questioned 11 people over the killing, nine of whom, aged between 20 and 28, remain on bail.