A man stabbed to death at a London Underground station was an Arsenal fan on his way to a match.
Tashan Daniel, 20, was fatally wounded in an "unprovoked attack" on the platform at Hillingdon station on Tuesday shortly before 16:00 BST.
He had been heading to the Emirates Stadium to see the Gunners face Nottingham Forest in the third round of the Carabao Cup.
The Prime Minister called the killing "a senseless act".
Writing on Twitter, the Prime Minister offered his "heartfelt condolences to the friends and family of Tashan".
He also confirmed he had been in touch with British Transport Police (BTP) following the incident.
It is the third murder investigation on the Tube network this year.
A spokesman for the Arsenal said: "Everyone at Arsenal Football Club is shocked and deeply saddened to learn of the tragic passing of Arsenal fan Tashan Daniel.
"Our thoughts are with Tashan's family and friends at this sad time."
Mr Daniel, who had recently celebrated his birthday, was on the platform waiting for a Piccadilly Line train into central London when he was attacked by two men, British Transport Police (BTP) said.
Attempts were made to save Mr Daniel who was pronounced dead inside a Tube carriage.
Det Ch Insp Sam Blackburn said no arrests had been made, but both suspects left the station in the direction of Auriol Drive and a knife was recovered nearby.
"Tashan did not deserve to lose his life during this senseless attack," he said.
"His family are completely broken by this news and we are doing everything we can to offer them support."
Extra officers are going to be carrying out patrols in the Hillingdon area, Det Ch Insp Blackburn added.
Mr Daniel was "the perfect kid" according to his running coach Courtney Clarke.
Mr Clarke told the BBC he "hasn't stopped crying" all day over the news of Mr Daniel's death.
"This shouldn't be happening," Mr Clarke said.
James Manley, Mr Daniel's team manager at Hillingdon Athletic Club, said: "We are all devastated by Tashan's death.
"He always went the extra mile for his team and community.
"He was ceaselessly kind and generous with his happiness. In return he was loved by all of us."
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."
The station was closed by Transport for London (TfL) while police searched the area.
It remained closed for the rest of Tuesday evening and reopened at 05:45 on Wednesday.
So far in 2019 more than 100 murder investigations have been launched across London by the Metropolitan Police and BTP.