Rangers midfielder Claudio Reyna has branded the Celtic supporter who appeared to mimic a plane during yesterday's Old Firm game as "disgusting".
The fan seemed to be taunting the US international over the terrorist attacks on New York and Washington last month. The Celtic supporter was caught on camera making the gesture.
Celtic are investigating the incident, which happened as Reyna prepared to take a corner.
Reyna told STV Scotland Today: "It's disgusting to think about it because of how horrible the tragedy was and how someone could stoop that low.
"I did honestly think about it afterwards and I was surprised. I thought that if anything it [the tragedy] would teach people to unite and not to act like that".
Reyna said the fan should be punished for his actions. "If they can punish the person they should because that's pretty awful or they should at least point it out to the fans because it's just uncalled for".
A spokeswoman for Celtic said the club would try to find the fan. She said: "It is absolutely intolerable and unacceptable behaviour and we will investigate to find the individual concerned".
She said the club would seek the help of the police and to ban the supporter from attending matches.
- PA