Liverpool say they have “substantial evidence” relating to a claim by Manchester City that a supporter spat at a member of their staff during their game at Anfield on Sunday.
City made an official complaint to Liverpool following the thrilling 2-2 Premier League draw.
It is alleged that the incident happened after City scored their first equaliser, and Liverpool have said their investigation includes looking at footage and collecting statements from supporters.
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"We are conducting a full investigation, working with all relevant parties including Manchester City, to gather evidence from individuals who witnessed the incident as well as analysing all available video footage," the club said in a statement.
"The evidence collected to date is substantial and, if requested to do so, will be handed to Merseyside Police given the potential for criminal charges. This case will also be subject to the club's official sanctions process.
"Anfield is our home and we will ensure it is a safe, enjoyable environment for everyone attending and will not allow unacceptable isolated behaviour to impact on our club's values and principles."
Man City head coach Pep Guardiola said after the game that the incident was "certain people - not just LFC", after also referencing the defacing of their team bus when they visited Anfield in 2018.
"I don't know what happened,” he told Sky Sports. “I heard something from the crowd - something not nice to staff. But I don't know what exactly. I didn't see the images.

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"I'm pretty sure that if it happened, what the people told me, I'm sure Liverpool will take measures about this person. Liverpool is much greater than this behaviour.
"Nothing will change my admiration for this club. It was the same story two, three years ago when we came here and they destroyed our bus - it is certain people, not just Liverpool FC.
"It's always a pleasure to come to this stadium. It's an honour for this atmosphere, how they support the team. That's why it's Anfield."
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