A 15-year-old was brutally murdered outside a party that had spiralled out of control just as police arrived.
Officers got to the community centre in Clockhouse Lane, Romford at 9.22pm on Saturday and were dealing with "a large number" of teenagers involved in various brawls.
Metres away at the junction of Lynwood Drive and Clockhouse Lane, Jordan Douherty was being beaten and stabbed in an attack that would end his life.
Investigators are trying to establish why the aspiring rapper was the only one injured among the teenagers who overran a 16th birthday party.
A massive crowd swarmed to the event after it was advertised on social media.
When some gatecrashers were asked to leave the packed venue, a number of fights broke out and police were called at 9.05pm.
Officers arrived at 9.22pm and they believe that the attack on Jordan began at 9.20pm.
The youngster, who performed drill music under the name Young Valenti as part of a group called C17, was targeted by more than five assailants in three attacks seconds apart, during which he was stabbed twice.
Detective Chief Inspector Larry Smith told the Press Association: "As far as we're aware he's just a young man who's just turned 15, who's gone to the party of a 16-year-old girl and as a result of that he was brutally murdered.
"There's nothing to suggest his involvement in drill music was a motive. Really at the moment nothing to suggest any motive for the attack.
"Clearly there was something to cause the individuals involved to focus their attentions towards Jordan but at the moment we're unclear as to what that is.
"For some reason Jordan was the focus of their attentions. There were a number of other people there, but clearly they wanted to assault and seriously injure Jordan."
Jordan is the youngest victim among the nearly 80 homicides that have taken place in London so far this year.
He was involved in drill music, an urban genre laden with references to violence and often used by gang members to taunt rivals.
The Met recently sought a court order on five youths forcing them to inform police before recording or performing their music after issuing threats and inciting attacks in their songs.
Detectives investigating Jordan's murder are examining CCTV footage and have appealed to witnesses who filmed the fighting on their mobile phones to pass the footage to police.
Mr Smith said: "We've spoken to a number of people who were there, but we're appealing to everybody that was in the community centre, in the vicinity of Clock House Lane, a pedestrian or driving past, please contact us with details of anything you heard or what you saw.
"You may think it is irrelevant but please let us make that decision.
"We're aware that people were videoing the events that evening, we're very, very keen to see any product that people have.
"Someone will have seen Jordan being assaulted and stabbed, and we're appealing to those people to come forward and tell us what they saw, and who they saw do what."
Three teenagers, aged 15, 16 and 17, were arrested on suspicion of murder in the aftermath of the attack but have since been released under investigation.
Anyone with information can call police on 0208 345 3985 or Crimestoppers anonymously on 0800 555 111 or online.