A 16-year-old boy was knifed to death in the street in a suspected drugs-related attack.
Hani Abou El-Kheir was set upon on the doorstep of one of London's most affluent neighbourhoods at 7pm on Sunday.
A gang of youths, reported to have been wielding swords and knives, left the youth bleeding to death as emergency services battled to save him in Lupus Street, Pimlico.
His mother Pauline Hickey was said to be "heartbroken" after it emerged the teenager had become known to drugs intervention workers.
Hani, named by his former school and family friends, was on the "periphery of having some sort of contact with gangs or activity such as drugs", a council drugs worker said. His name had been passed to drugs workers.
But ground-level intelligence had linked him as mixing with others involved in the underworld and he was due to get a visit from outreach workers if his name re-emerged.
David Savizon, who works for the authority's Your Choice anti-gangs programme, said: "Unfortunately I wasn't able to meet him." The teenager's name had been flagged up to drugs workers on Wednesday, Mr Savizon, 34, said, adding: "He wasn't deemed significant in terms of being in a gang. He was mentioned as someone on the periphery."
Detective Chief Superintendent Paul Rickett said: "This incident is particularly sad given the large amount of progress that has been made in tackling youth violence in Westminster over the last year; there has been over a 60 per cent reduction in the numbers of incidents of serious violence involving youngsters, this equates to more than 90 fewer victims. The number of knife crime victims on the borough is down by 57 per cent.
"This case is currently being investigated by detectives from the Homicide and Serious Crime Command and an incident room has opened which I urge people to contact if they have any information. The number to call is 020 8345 3715; or to remain anonymous please call Crimestoppers on 0800 555 111."
Scotland Yard later confirmed the victim's name as Hani Hicham Abou El-Kheir and said a post-mortem examination would take place on Tuesday. Detectives continued to appeal for witnesses and said Hani was assaulted by "several youths". The force said they were "retaining an open mind regarding any motive at this stage".