A Sri Lankan teenager was stabbed to death in Wembley on Saturday.
The youth - Asan Ratnasegaram, 18, from Wembley - was attacked while travelling with two other Sri Lankan men in a red saloon car.
Police are appealing for witnesses to the stabbing, which they believe happened as the men were stopped at the junction of Ealing Road and Carlyon Road, Wembley, at about 1300 BST.
They were attacked by an group of men - thought to be of Sri Lankan appearance - who pulled up behind them, also in a red saloon.
The suspects drove off. New Scotland Yard said it was not known at this stage how many there were.
Mr Ratnasegaram was taken to Central Middlesex Hospital, where he was pronounced dead.
A post-mortem examination on Sunday said he died of a stab wound to the chest.
Officers from the Serious Crime Directorate, led by Detective Chief Inspector Norman McKinlay, are investigating the incident as a murder.
It is thought Saturday's incident may be the latest in a series of violent clashes between Sri Lankan Tamil gangs.
In June two Sri Lankan men were killed - one in Wembley and one in Ilford, east London.
Violence between the gangs has been increasing over the last three years.