Monkey chants and apparent Nazi salutes overshadowed England’s 6-0 win in Sofia on Monday, with the match halted twice during the first half due to the abuse.
“Six people have been detained and another three are being actively sought,” Sofia police chief commissioner Georgy Hadzhiev told journalists.
Hadzhiev added that so far 15 people have been singled out via CCTV at the Vasil Levski National Stadium as being suspected of directing abuse against England’s black players.
Police in the central city of Plovdiv said they were calling in for questioning 13 others, who were allegedly also part of the group of black-clad men who sparked trouble at the game, according to public BNR radio.
Penalties for the offences carry brief detentions, fines and bans from sporting events.
In total, 15,000 people watched the match at a stadium already partially closed after racist incidents during games against Kosovo and the Czech Republic in June.
England manager Gareth Southgate told reporters after the game that his side had been ready to walk off the pitch if the abusive behaviour continued but players decided to complete the match.
The incident sparked a storm of angry reactions from fans, media and officials in both countries.
British Prime Minister Boris Johnson condemned the “vile” racism, while his Bulgarian counterpart Boyko Borisov forced the country’s football federation chief Borislav Mihaylov to offer his resignation on Tuesday.
In a statement late Tuesday, Bulgarian national team manager Krasimir Balakov offered his apologies to the England player.
The scandal divided Bulgarians — while many apologised in posts on social media about what they said was a pervasive problem in the country, others refused to be identified with the offenders and condemned British media headlines that called all Bulgarians “racists” and “animals”.
UEFA announced that it was launching a probe into the behaviour of both the Bulgaria and England fans. Its president Aleksander Ceferin stressed the commitment of European football’s governing body to root out the “disease” of racism.
