Police said on Monday that they were working to track down the person responsible for throwing a coin at Manchester United's Rio Ferdinand during Sunday's Manchester derby in the English Premier League.
Ferdinand was left with blood streaming down his face from a cut above his eye after being struck by a coin as he celebrated Robin van Persie's injury-time winner in United's 3-2 victory at Manchester City.
Greater Manchester Police said that they had made 13 arrests and charged nine people over offences that occurred before, during and after the match.
"To have just 13 arrests for a crowd of this size and a match of this proportion is a testament to the policing operation we put in place," said Chief Inspector Steve Howard.
"Despite fierce rivalry and high tension, there was no major disorder. However, we will continue to investigate the coin-throwing incident and are determined to work with the club to bring the perpetrator to justice."
The Football Association (FA) are also investigating the incident.
Among the people charged were two men accused of entering the field of play during the game.
City goalkeeper Joe Hart had to restrain one young fan from getting at Ferdinand after the coin-throwing incident, prompting the United defender to thank his one-time England colleague on Twitter.
A 30-year-old man was also charged with what police said was a "racially aggravated public order" offence.
All the people charged are due to appear before magistrates in Manchester on January 4 next year.
United's victory took them six points clear of defending champions City at the top of the Premier League table.
"what a result, 3points! Sweet! Whoever threw that coin, what a shot! Can't believe it was a copper 2p....could have at least been a £1 coin!"
- Rio Ferdinand responds to the coin attack on Twitter (@rioferdy5)
FA chairman David Bernstein said the crowd trouble that marred the game was "deplorable" and called for strict punishments to be meted out to those responsible.
"It is deplorable to see those incidents and to see Rio Ferdinand with blood on his face is absolutely terrible," he told Sky Sports News.
"I think it's disturbing that we're seeing a recurrence of these types of incidents. We've had racial abuse issues, the odd pitch incursion, things being thrown at players -- it's very unacceptable and has to be dealt with severely."
He added: "It's a difficult social problem. I think there's a copycat thing: something happens and other people copy it and this sort of thing can spiral.
"To my mind it's for the FA, the whole game of football and the authorities to work together to deal with this most severe matter.
"I believe that if necessary these people need to go to the court and be banned for life, if they're found out."
Meanwhile, Professional Footballers' Association (PFA) chief executive Gordon Taylor said there was a case to be made for erecting nets to protect players from missiles thrown by supporters.
"I think you've got to give consideration to possibly, as has been suggested, some netting in vulnerable areas, be it behind the goals and round the corner flags," he told BBC Radio 5 Live.