Uefa has opened an investigation into the racial abuse directed at Liverpool's Djibril Cisse during the Champions League qualifier in Bulgaria.
Cisse, who scored in Liverpool's 3-1 Champions League win, was subjected to monkey noises from a section of fans.
Reds boss Rafael Benitez said after the match that Uefa's official delegate Trygve Borno had heard the abuse.
"Uefa is aware incidents of a racist nature have taken place," a Uefa statement said.
The case was opened after Uefa received the official delegate's report from the CSKA Sofia match.
Uefa's control and disciplinary body will consider the case at its next scheduled meeting on 18 August.
"Always it is a pity to hear such things but the Uefa official has heard and it has gone in his report," said Benitez.
"The atmosphere in terms of support was good but the monkey noises are a pity."
On a claim from a Bulgarian journalist that Cisse provoked the crowd, Benitez said: "That is stupid. To score goals is not to provoke the fans."
Liverpool press officer Ian Cotton confirmed: "The Uefa official has told our secretary Bryce Morrison that he has heard the monkey noises and he will include that in his report."