A man died and another was left seriously injured after they were stabbed in an attack in Amsterdam.
Cleveland Police said they are in regular contact with police in Holland, who have started a murder inquiry.
The two men are both thought to be from Teesside but police said they had not been able to contact family members.
A police spokesman said they could not confirm the two men had been in Holland to watch Middlesbrough's goalless draw with AZ Alkmaar on Thursday.
Det Ch Supt Mark Braithwaite, from Cleveland Police, said the two men were in a bar called Pinocchio's in central Amsterdam in the early hours of Friday when it appeared there was a confrontation with a third man.
Briton arrested
He said the three went outside where there was a stabbing in the street.
Two senior detectives from Cleveland Police are in regular contact with the Dutch authorities and there is a possibility they will fly out to Amsterdam over the weekend.
Police are appealing for anyone who was in the bar to contact them.
Middlesbrough FC media and communications manager, Dave Allan, said: "Cleveland Police are keeping the football club fully informed as more information becomes available."
Police said in a separate incident in Alkmaar, a Dutch man suffered serious injuries in an attack with a glass. A British national was arrested in connection with the incident.
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