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 told the PA news agency that 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 that 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.
"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."
