Middlesbrough have suspended two supporters after an investigation into an alleged racist incident at Saturday's 2-1 victory over Leicester.
Leicestershire Police have now opened a criminal investigation following the reports from the King Power Stadium.
Boro said they would co-operate with anti-racism charity Kick It Out, Leicester and relevant authorities.
"We have a zero-tolerance approach towards racism and all forms of discriminatory behaviour," they added.
"We will take the strongest action possible. Our club is an inclusive football club where everyone should feel safe and welcome."