Liverpool have confirmed they have gathered "substantial evidence" on a complaint of a serious alleged incident close to the dugout at Anfield on Sunday.
And a spokesperson revealed they are ready to pass the evidence to police, if requested to do so, given the potentially criminal nature of the offence.
The club issued an update on Monday night, responding to the official complaint from City after their encounter at the weekend, when the visitors suggested one of their back room staff had been spat at by a supporter.
The statement read: "Following a complaint of a serious alleged incident near the dugout area at Anfield during the second half of Sunday's game against Manchester City, we are conducting a full investigation.
Liverpool have confirmed they have gathered "substantial evidence"
"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."
Spitting directly at another person can be a criminal offence, and Liverpool are ready to work with police to identify any offender. The club can also impose a lifetime ban any fan responsible.
It is understood a number of witness statements have been collected from those seated around the dugout area, and Liverpool have worked closely with City staff to gather more statements, and full video footage of the incident.