A man believed to have attacked Sheffield Wednesday goalkeeper Chris Kirkland during a match against Leeds has been identified by police.
The suspect, who has not yet been arrested, is thought to have run on to the field from the away end in the 77th minute, seconds after Leeds had equalised in the Yorkshire derby and knocked Kirkland to the ground.
He then returned to the stand while trouble continued around him, with a number of other travelling supporters invading the pitch.
The Football Association is also investigating crowd trouble at the match and is awaiting the referee's report.
Bottles and seats were believed to have been thrown, South Yorkshire Police said.
Chief Superintendent Jason Harwin added: "Such incidents are not acceptable and won't be tolerated. We will be working with both clubs to identify and quickly bring to justice the persons responsible.
"Thankfully, such incidents are a rarity and we see a minority spoil it for the majority of genuine supporters."
Detective Inspector Mark Monteiro, said: "With the public's assistance we have clearly identified the offender who ran on to the pitch during last night's game between Sheffield Wednesday FC and Leeds United FC and who is suspected of assaulting the Sheffield Wednesday goalkeeper.
"We are dealing with the complaint made by the player and we are currently taking statements, collating evidence and making inquiries to locate and arrest the offender.
"We are also using CCTV footage from around the ground to identify suspects who may have been responsible for other offences before, during or after the match."
Former England international Kirkland required treatment after the attack, but was able to continue. The game ended 1-1.
In a post-match interview, Sheffield Wednesday manager Dave Jones described Leeds fans as "vile animals", adding: "You talk about racism, the chanting, that's a form of racism.
"Then (for the Leeds players) to go and wave at them ... not in my book. You don't wave at them after that."
Riot police were present at the game and tried to diffuse ugly scenes in the away end.
Describing the scenes as an "absolute disgrace", Leeds manager Neil Warnock called for Kirkland's assailant to be given a jail sentence.
He said: "They should get the guy and prosecute him and put him in prison. He (Kirkland) went down like a ton of bricks but that doesn't make a difference. Nobody should be on the pitch doing that.
"I am not proud of being Leeds manager when I see that. I don't mind the rivalry between us - there was a great atmosphere.
"To see things like that on the field of play there is no place for it - I am absolutely embarrassed."
The visiting club issued a statement after the game, saying: "Leeds United Football Club would like to publicly apologise and condemn the action of the fan who came on the pitch and attacked Sheffield Wednesday goalkeeper Chris Kirkland.
"The club will fully co-operate with the police and the football authorities in identifying the individual concerned."