A second youth has been arrested on suspicion of murder after a teenager was stabbed to death in west London on Friday morning, police said.
The victim, 17, was rushed to a central London hospital, where he was later pronounced dead.
Officers were called to Crown Street following the stabbing in Acton at 11:50am.
A 16-year-old male was arrested on Friday evening on suspicion of murder and he remains in custody at a south London police station.
A 17-year-old male who was arrested near to the scene of the murder shortly after the incident remains in custody at a west London police station. He has also been arrested on suspicion of murder.
Officers believe they know who the dead youth is, although they await formal identification. Next of kin have been informed.
A post-mortem examination is scheduled to take place at midday today at Fulham Mortuary.
Detective Chief Inspector Rebecca Reeves, who is leading the investigation, said: "Crown Street would have been a busy at the time of the stabbing and we are appealing for any witnesses, or anyone with any information, that was in or around the area to contact us.
"We are particularly keen to hear from anyone who saw or heard an argument outside a cafe on Crown Street."
Any witnesses or anyone with any information are asked to call the incident room on 020 8721 4868, or contact Crimestoppers anonymously on 0800 555 111 or via crimestoppers-uk.org.