A man found fatally stabbed on a London bus as been named as Archie Sheppard, 48.
Detectives said Mr Sheppard, from Neasden, north-west London, was subject to a "violent and sustained attack" on the top deck of a Route 189 bus before being discovered by a passenger.
Police and paramedics were called to the bus in Gloucester Place, Marylebone, at 12.10am on Friday, but he was pronounced dead at the scene.
Scotland Yard appealed for help in searching for a white male seen with Mr Sheppard shortly before he boarded the bus in Brent Cross, north London, at 11.13pm on Thursday.
Detective Chief Inspector Noel McHugh, said: "Our inquiries so far have established that Mr Sheppard and the man we are seeking to trace were seen talking together while waiting at the bus station.
"They both boarded a Route 189 bus and made their way to the upper deck where they sat separately.
"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.
"Approximately 40 minutes into the journey, Mr Sheppard was subjected to a violent and sustained attack."
The attacker then fled the bus as it reached Gloucester Place and the junction with Montagu Place, heading in the direction of Marble Arch, police said.
"Mr Sheppard was discovered when a passenger made their way to the upper deck and an ambulance was called. He was taken off the bus for urgent medical intervention but sadly died at the scene," the detective said.
A post-mortem examination is scheduled to take place on Saturday and Mr Sheppard's next of kin have been informed.
Several witnesses are helping police, although no arrests have been made and police appealed to anyone who was on the bus to contact them.
Anyone who witnessed the incident or the events leading up to the murder has been asked to contact the incident room on 020 8785 8099 or via Twitter @MetCC
Information can also be reported anonymously to Crimestoppers on 0800 555 111 or via the crimestoppers-uk.org website.