The hooligan who ran on to the pitch and assaulted Sheffield Wednesday goalkeeper Chris Kirkland has been identified by police.
The suspect, who has been named on social media sites but not by police, has yet to be arrested after former England goalkeeper Kirkland was shoved in the face amid ugly scenes at Hillsborough.
The Football Association is also investigating crowd trouble at the Yorkshire derby and is awaiting the referee's report.
Kirkland was assaulted by a Leeds fan after the visitors had equalised.
The shocking scenes were broadcast live on Sky Sports and fans were quick to condemn the trouble on Twitter and Facebook. That led to a suspect being named on sites, and a mobile phone number being circulated.
South Yorkshire Police's senior investigating officer, Detective Inspector Mark Monteiro: "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.
"I'd like to thank the supporters and general public who have provided information and assisted with our inquiries."
An FA spokesman: "The FA condemns the scenes at Hillsborough last night. We have begun an immediate investigation and have been in contact with both clubs.
"We would call for the authorities to identify and bring before the courts anyone who was involved in any form of criminal disorder."
Leeds United statement: "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.
"After the week football has endured there is no place for this type of behaviour and the majority of Leeds supporters will be ashamed of his actions."
Anyone with information is asked to call police on 101 or Crimestoppers on 0800 555 111.