The Bulgarian government says it has detained four football fans linked to making racist gestures, including Nazi salutes, during a European Championship qualifying match against England.
Bulgarian fans also directed monkey noises at England's black players during the match, which was halted twice in an effort to stop the racist abuse. England won the match 6-0.
The Ministry of Interior says police have "detained four people, whose involvement in what happened is being clarified".
The ministry says any evidence collected will be handed over to prosecutors.
In the fallout from Monday's game, the president of the country's football federation has resigned and the Bulgarian special police forces have raided the federation offices.
England footballers slam Bulgaria
Manchester City star Raheem Sterling, Manchester United forward Marcus Rashford and Aston Villa defender Tyrone Mings were targeted in Sofia by many Bulgarian fans who called them monkeys.
"Feeling sorry for Bulgaria to be represented by such idiots in their stadium. Anyway, 6-0 and we go back home, at least we did our job. Safe travel to our fans, u guys did well," Sterling tweeted early Tuesday.
"Very proud moment for me and my family tonight to make my England debut. The unfortunate incidents in the game were handled as well as possible & proud of how we handled it," Mings tweeted.
Rashford said that this type of behavior is unacceptable and should not happen in this day and age.
"Also been told what the Bulgaria captain did at half-time. To stand alone and do the right thing takes courage and acts like that shouldn’t go unnoticed.
#NoToRacism," Rashford said.
Bulgaria coach apologises to England
Bulgaria coach Krasimir Balakov apologised to the England team after fans taunted their black players with Nazi salutes and monkey chants.
While Balakov said after the game he had not heard any racist abuse from the stands, in an email to reporters late on Tuesday he apologised for the fans' behaviour.
"I strongly condemn and reject racism as a norm of conduct that contradicts modern human relations," Balakov said.
"This is a prejudice that comes from the past that must be eradicated forever.
"I want to say something very clear: given that there have been insults on such a basis, I, as a coach of the national team, apologise to the English footballers and all those who feel hurt."
Bulgarian Football Union President Borislav Mihaylov resigned on Tuesday, a few hours after the Balkan country's Prime Minister Boyko Borissov called for the former international goalkeeper to step down.
Mihaylov had previously defended Bulgarian football from accusations of racism and criticised England for what he saw as a “fixation” on potential incidents that could raise tension.
UEFA, European football's governing body, has repeatedly vowed to eliminate racism and discrimination from the game.
In recent years, UEFA released a video to raise awareness about racism, with many football stars such as Cristiano Ronaldo, Lionel Messi, and Zlatan Ibrahimovic joining the campaign.