Nottinghamshire Police arrested a man who ran onto the pitch during Nottingham Forest's 4-1 victory over Leicester City in the FA Cup on Sunday.
"Investigations are in the very early stages and our enquiries into the circumstances surrounding the incident are ongoing," chief inspector Neil Williams said.
The man confronted Nottingham Forest players who were celebrating Joe Worrall's goal in the 32nd minute.
Leicester City issued a statement saying it was "appalled by such behaviour and offers its apologies to Nottingham Forest and its players whose safety was compromised." The club also said the supporter is facing a lifetime ban from Leicester City matches at King Power Stadium and any away ground.
Per the Guardian's Peter Lansley, the supporter's actions are also the subject of an investigation by England's Football Association.
The incident didn't overshadow arguably the most shocking result from the fourth round of the FA Cup.
Philip Zinckernagel gave Forest the lead in the 23rd minute, and Leicester was just getting play restarted from the kickoff when Brennan Johnson doubled his club's lead.
Emirates FA Cup @EmiratesFACup
IT'S THE STAR BOY 🤩<br><br>Brennan Johnson has scored straight from kick-off! ⚡️<a href="https://twitter.com/NFFC?ref_src=twsrc%5Etfw">@NFFC</a> <a href="https://twitter.com/hashtag/EmiratesFACup?src=hash&ref_src=twsrc%5Etfw">#EmiratesFACup</a> <a href="https://t.co/gQMZoxyBln">pic.twitter.com/gQMZoxyBln</a>
Staring at a 3-0 deficit, Kelechi Iheanacho's opener for Leicester provided little consolation in the 40th minute, and Djed Spence added a fourth for Nottingham Forest in the 61st minute.
Forest will play Huddersfield Town on March 2 in the next round of the FA Cup.