UEFA bosses have to wait for an official report before deciding whether to punish Liverpool for the Real Madrid bus attack.
Los Blancos overcame pre-match drama to hold the Reds draw with a goalless draw and advanced to the semi-final against Chelsea with a 3-1 final victory.
Uefa In-game observers have to decide whether or not to include the incident in their report.
Videos and images of a broken window on the side of the luxury have appeared on social media.
A Merseyside Police statement wrote: “An investigation is underway after damage has been caused to Real Madrid tonight’s team bus as it approached Anfield Stadium.
“Such incidents will not be tolerated and we are working with Liverpool and looking at CCTV and body footage.”
When one of Real’s buses was parked at the end of the Kop courtyard after the team got off the train, it was clear a double glazed window had been smashed.
Liverpool employees can be seen cleaning the broken glass with the participation of Merseyside police.
Hundreds of Liverpool fans had previously been spotted lining up on Anfield Street before the game began.
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A Liverpool spokesman said of the incident: “We strongly condemn the actions that resulted in the Real Madrid team’s bus being damaged during the trip to Anfield tonight.
“It was the completely unacceptable and embarrassing behavior of a few individuals.
“We sincerely apologize to you for any suffering you have caused. We will work together with Merseyside Police to verify the truth and identify those responsible.”
Europe’s governing body fined Liverpool £ 17,000 after the Man City manager was thrown bottles, cans and flares in April 2018.