Bulgarian police have arrested six people over racist abuse hurled at England players during Monday's Euro 2020 qualifier in Sofia.
The authorities added that they are trying to identify more suspects.
England routed Bulgaria 6-0 on Monday during their clash in the Vasil Levski stadium. But the match was halted twice in the first half under the new three-step procedure from Europe's football governing body, UEFA, due to home supporters — some wearing masks — taunting black English players with Nazi salutes and monkey chants.
England's Football Association immediately condemned the "abhorrent racist chanting" levelled at the national team as "unacceptable".
The UEFA has since opened disciplinary proceedings against Bulgaria on charges including "racist behaviour", "throwing of objects", and "disruption of national anthem".
The incident has also led to the resignation of Bulgaria's football union chief Borislav Mihaylov.
Meanwhile, team manager Krassimir Balakov, who initially downplayed the incident, has since released a statement on Facebook to "sincerely apologise to the English footballers and to all those who felt offended."
"I condem all forms of racism as an unacceptable behaviour that contradicts normal human relations," he added.