Four men have been detained by Bulgarian police in relation to the racist abuse of England players on Monday night.
The Bulgarian ministry of the interior said the operation to identify individuals involved in the abusive chanting was still ongoing.
England's black players were targeted throughout the match, and the Bulgarian Football Union has been charged by UEFA over the racist behaviour of its fans.
A spokeswoman for the ministry said: "The operation of Sofia metropolitan police is still ongoing. At this stage four males are detained.
"The work is ongoing to establish whether they were involved in this incident. The operation continues during the day."
The spokeswoman also confirmed special forces from the ministry's general directorate for combating organised crime had raided the BFU offices on Tuesday.
The BFU's president, former national team goalkeeper Borislav Mihaylov, announced his resignation in the wake of Monday night's racist chanting.
Under the UEFA regulations, the BFU could face a full ground closure for their next competitive home match and a fine of 50,000 Euros, but Aleksander Ceferin, the president of European football's governing body, spoke on Tuesday about 'waging war' on racism.
Under Article 14 of UEFA's disciplinary code which relates to racism, there is scope to impose a tougher sanction such as points deduction or even disqualification "if the circumstances of the case require it".
FIFA president Gianni Infantino also suggested any penalties imposed would be extended worldwide.
He said football needed "new, stronger and more effective ways to eradicate racism in football" and called for life bans from stadiums for any individuals found guilty of racist behaviour.
Ceferin said on Tuesday: "UEFA's sanctions are among the toughest in sport for clubs and associations whose supporters are racist at our matches. The minimum sanction is a partial closure of the stadium - a move which costs the hosts at least hundreds of thousands in lost revenue and attaches a stigma to their supporters.
"UEFA is the only football body to ban a player for 10 matches for racist behaviour - the most severe punishment level in the game. Believe me, UEFA is committed to doing everything it can to eliminate this disease from football. We cannot afford to be content with this; we must always strive to strengthen our resolve.
"More broadly, the football family - everyone from administrators to players, coaches and fans - needs to work with governments and NGOs to wage war on the racists and to marginalise their abhorrent views to the fringes of society.
"Football associations themselves cannot solve this problem. Governments too need to do more in this area. Only by working together in the name of decency and honour will we make progress."
Football Association chairman Greg Clarke said on Monday night that a stringent review was required into the events in Sofia and that "zero tolerance" had to be shown towards racist behaviour.
He told ITV Sport: "UEFA are going to have to think very carefully about the level of abuse they're prepared to let players tolerate and they're going to have to decide who they are going to make an example of one day, but that's after a thorough examination of the facts."