Bulgaria coach Krasimir Balakov said he did not hear any racist chanting during his side's 6-0 Euro 2020 qualifier defeat by England on Monday.
Play was halted twice in the first half because of abuse from the stands and England players had the option to walk off the pitch, but chose to continue.
Balakov said he would be "truly sorry if it did turn out to be true" but said there had been no such problems before.
"I personally did not hear the chanting," Balakov said.
"I saw the referee stopped the game but I also have to say the behaviour was also not only on behalf of the Bulgarian fans but also the English fans, who were whistling and shouting during the Bulgarian national anthem.
"During the second half they used words against our fans which I find unacceptable."
The Vasil Levski Stadium was subject to a partial closure for this match after Bulgaria were sanctioned for racist behaviour during qualifiers against Kosovo and the Czech Republic.
"This has not happened to us before," said Balakov, whose translator added that the disciplinary measures imposed on Bulgaria by Uefa were not because of racist chanting but "because of a banner of an organisation that is not forbidden by Bulgarian law".
"We've had this problem ever since England were about to come to Bulgaria and all I've heard for three weeks is people talking about anything else but football. I don't think this was the proper manner to prepare and play a football game.
"If this turns out to be true we are truly sorry and we as the Bulgarian Football Union and the Bulgaria national team are working very hard."
Bulgaria captain Ivelin Popov had been seen talking to a group of Bulgaria supporters at half-time, prompting England striker Marcus Rashford to praise his "courage".
Balakov says he was unaware Popov spoke to some fans at half-time, adding: "It is probably because the fans were unhappy with the way the team was performing."