Violent footage of people being attacked at random has been found on the mobile phone of a 17-year-old boy.
The phone was confiscated as part of the investigation into the killing of 18-year-old Triston Christmas in 2005.
Triston's mother said a mobile phone video recording of him as he lay dying had been passed around his classmates.
Police never found footage of Triston, but did find 14 other assaults, in north or east London, and want the victims to come forward.
They think the assaults were deliberately staged for filming.
One, titled "Bike", shows at least two teenagers laying in wait for a passing cyclist then jumping out and knocking a woman from her bike.
Others show people being punched, kicked, slapped and mugged by a laughing, taunting gang.
Triston died after being punched by 18-year-old Gary Roper in January 2005, for which Roper was sentenced to 30 months' youth detention.
Tristan's mother Siobhan Christmas, 43, says she has seen footage of her son lying on the ground with blood bubbling out of his mouth, though it has never been recovered by police.
The video was taken by one of the crowd who saw the incident. It was being passed round computers and phones at Triston's school.
"To all the bullies out there, would they like their father or mother, sister or brother to watch footage of them dying on a phone? I had to watch footage of my son lying on the ground, twitching and groaning.
"My son is dead because of this kind of behaviour. I have to live day in and day out without my beautiful vibrant boy."
Police investigating reports that some youngsters had videoed the attack seized several mobile phones.
Det Ch Insp Ellie O'Connor, who is investigating the videoed assaults at Barking and Dagenham CID, said: "We now need to identify the victims of these assaults in order to pursue a criminal investigation.
This picture shows a woman being knocked off her bike
"Every phone incident captured is incredibly violent which is why we are classing these as serious assaults.
"There is nothing amusing about the attacks and every victim is visibly distressed. We are keen to trace each victim to ensure that those responsible do not go unpunished."
Faces have been pixellated so the victims cannot be identified by anyone else, but police say they will recognise themselves.
They also want anyone who recognises the areas filmed to get in touch.