A man was stabbed to death in front of scores of passers-by in Oxford Street yesterday.
The 22-year-old was murdered after leaving a McDonald's restaurant. He was taken by ambulance to hospital, where he was pronounced dead.
There were no immediate arrests, but detectives were sifting through interviews with the many eyewitnesses to the stabbing for clues about what happened.
The attack took place outside the McDonald's near Oxford Circus Tube station at just before 5pm.
There was initial speculation that the victim could be a security guard at one of Oxford Street's stores. But a source said he was a member of the public who had been in the restaurant before the assault.
One witness, who did not want to be named, said: "I just heard the screaming and then saw a couple of black guys run past us."
A McDonald's spokeswoman said none of the fast food chain's staff were involved in the incident.
She went on: "I know that one member of staff who was actually on their way to work came up and saw the crowd outside the restaurant. He has given evidence to the police but as far as I know no one else saw anything.
"But obviously all members of staff that were on duty at the time of the incident will speak to the police as part of the ongoing investigation."
McDonald's refused to comment further, saying it did not wish to "prejudice any future case".
The victim has not yet been formally identified, although police believed they knew his identity.
Scotland Yard said it was too early to say whether he was with friends at the time or whether the attack was gang-related.
A post-mortem examination will be carried out in due course.