Four Bulgarian football fans suspected of subjecting black England players to racist abuse have been detained following police raids.
England's 6-0 Euro 2020 qualifier win over Bulgaria in Sofia was stopped twice in the first half following racist chanting by home supporters.
"Work is still ongoing to identify others," a statement from Bulgaria's Ministry of the Interior said.
Bulgaria's football chief Borislav Mikhailov resigned on Tuesday.
The English FA and Uefa have condemned the actions of the 'ultras' section of Bulgaria fans, with Aleksander Ceferin, president of the sport's European governing body, calling for "football family and governments" to "wage war on the racists".
England midfielder Jordan Henderson called the behaviour of Bulgaria fans "disgusting", after they were warned for making Nazi salutes and monkey noises.
"I obviously wasn't happy with the situation that we were in - it wasn't nice to be involved in and it shouldn't be happening in 2019," he said.
Team-mate Tyrone Mings, who was making his international debut on Monday, said he heard racist chanting "clear as day" during the pre-match warm-up in Sofia.
A number of players posted on social media following the game, thanking travelling fans for their support and expressing their pride in the performance despite the abuse.
"Not an easy situation to play in and not one which should be happening in 2019," said striker Marcus Rashford. "Proud we rose above it to take three points but this needs stamping out.