Celtic have vowed to take action against a group of fans filmed singing an offensive song about murdered soldier Lee Rigby.
The club said it was "appalled" by the video showing men in Celtic strips apparently celebrating the death of the 25-year-old.
Fusilier Rigby's mother said her family was "hurt, traumatised and deeply shocked" by the incident.
The video is thought to have been filmed in Sunderland where Celtic were playing a friendly on Saturday.
More than 20 football fans were arrested after trouble broke out ahead of the match.
A statement from Celtic said: "Clearly everyone at Celtic is appalled by these events.
"Such behaviour in no way represents Celtic Football Club or our supporters.
"We understand this incident is now subject to an ongoing police enquiry.
"Let us be clear, in the event that any individuals are identified as being responsible, Celtic will take the strongest possible action."
The solider was murdered by Michael Adebolajo and Michael Adebowale outside Woolwich Barracks in London in 2013.