Detectives investigating the murder of teenager Hani Abou El Kheir want to speak to a taxi driver who picked up three suspects.
Hani, 16, was stabbed to death on January 27 in Pimlico, central London, by a group of people believed to have been wielding knives and swords.
Police were called to Lupus Street at 6.50pm, and Hani was rushed to King's College Hospital where he later died.
Detectives think three suspects were picked up in a taxi near Victoria station at 6.59pm. They got into a silver London taxi on Hudson Place, which then turned left onto Wilton Road. It is believed to be a T2 or T4 model with colour-coded bumpers and black trim around the door and bumper.
On Sunday night four people arrested in connection Hani's murder were released on bail. A man aged 48, a woman of 47 and two girls aged 16 and 17, who were arrested on Saturday on suspicion of assisting an offender were bailed to a date in March, Scotland Yard said. Two men aged 18 and 19 and two boys aged 16 and 17 arrested on suspicion of murder on Saturday remain in custody at various police stations in east London.
Detective Chief Inspector Neil Attewell said: "We are exploring numerous lines of inquires, with this latest development being one of them. I am appealing to the driver of the silver taxi seen in Hudson Place at 6.59pm on that Sunday to come forward. Are you that taxi driver? Where did you drop the passengers off and did they discuss anything of significance whilst in the taxi? I am also seeking the assistance of the wider taxi community. With a taxi rank close by there would have been other drivers who may have witnessed the suspects getting into the silver taxi."
Police believe at least 10 people were involved in the murder and are keeping an open mind regarding a motive.
A post-mortem examination on Tuesday found Hani died from multiple stab wounds.
The arrests on Saturday bring the total number held to nine. On Thursday a 20-year-old man was arrested in north London on suspicion of murder and bailed until late March.
Witnesses or anyone with information are asked to contact the incident room on 020 8345 3715 or Crimestoppers on 0800 555 111.