A man stabbed to death on a bus in Marylebone has been named by police as 48 year-old Archie Sheppard, from Neasden.
Mr Sheppard was fatally wounded on the 189 bus in Gloucester Place at about 00:10 BST on Friday.
Detectives believe he boarded the bus at Brent Cross bus station at 23:13 BST but are investigating when and where the stabbing took place.
He was treated by paramedics but pronounced dead at the scene.
No arrests have been made and police are continuing to appeal for witnesses to come forward with any information.
Detectives have established Mr Sheppard was attacked on the upper deck of the bus.
They want to trace a man who boarded the bus with Mr Sheppard and also sat on the upper deck, although not with him. Police initially described the suspect as black but later confirmed the man they are looking for is white.
Det Ch Insp Noel McHugh said: "Despite the late hour, Brent Cross bus station is a busy terminus and there would have been a number of people around who would have seen Mr Sheppard waiting for the bus.
"We are exploring the possibility that Mr Sheppard may have known his attacker and it is vital that anyone who saw the two men talking prior to boarding the bus contacts us."
The Met Police said Mr Sheppard was subjected to "a violent and sustained attack" approximately 40 minutes into his journey,
They believe his attacker fled from the bus at Gloucester Place at the junction with Montagu Place heading in the direction of Marble Arch.