"We are treating this as a terrorist investigation," Chief Constable Ian Hopkins told reporters, according to RIA Novosti.
Earlier, police announced that the investigation was led by counter-terrorist specialists, but they were "keeping an open mind".
A woman and two men were injured in the attack, one of whom has been identified as a British Transport Police officer.
The incident took place in Manchester's Victoria Station, which is situated close to the Manchester Arena, where a suicide bomber killed 22 people at a concert in 2017.
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