A man stabbed to death at a London Underground station was a football fan on his way to a match, the BBC has been told.
The Arsenal supporter was killed in a “unprovoked attacked” on the platform at Hillingdon station on Tuesday.
He had been on his way to the Emirates Stadium to see the Gunners face Nottingham Forest in the third round of the Carabao Cup, a source said.
It is the third murder investigation on the Tube network this year.
No arrests have been made over the attack, which Det Supt Gary Richardson described as “a shocking act of violence”.
He said the British Transport Police (BTP) investigation was in its “early stages”.
“We believe a group of young men were involved in an altercation on the platform before one of the men received a fatal stab wound,” he said.
Police were called to the station in west London shortly before 16:00 BST and the victim was pronounced dead at the scene.
Danielle Foster, who was driving past Hillingdon station at the time of the stabbing, said upon “hearing so many sirens, I knew something terrible had happened”.
“Lots of people were being turned away from the station as it had been closed,” she said, adding: “Then the police helicopter began circling the scene.”
Hillingdon station was closed by Transport for London (TfL) while police searched the area.
The station has since re-opened.
So far in 2019 more than 110 murder investigations have been launched across London by the Metropolitan Police and BTP.