Spanish prosecutors said on Friday they had opened an inquiry into Atletico Madrid's supporters over racist chants aimed at Real Madrid attacker Vinicius Junior ahead of the clubs' La Liga derby clash.
The investigation was opened following a complaint by an anti-racism organisation over the chanting which erupted from a group of Atletico fans at their Wanda Metropolitano stadium before and during Sunday's match.
"You are a monkey, Vinicius, you are a monkey," they could be heard chanting in footage recorded by Cope radio as the Real Madrid coach arrived at the stadium.
It said there were "hundreds" of fans involved.
Further chanting occurred during the match, with Atletico fans throwing things, including lighters, at Vinicius Junior and his Brazilian compatriot Rodrygo as they celebrated a goal.
"Madrid prosecutors have opened an investigation into the racist-tinged chanting which was heard both inside and outside Atletico Madrid's stadium against a Real Madrid player during their match on Sunday 18 September," a statement said.
"Prosecutors have asked the police to analyse the recordings of these incidents in order to identify the people who uttered these racist insults in order to see if they had any ties with violent or extremist groups," it said.
They had also asked the police to investigate whether the chanting was repeated inside the stadium, and have asked Atletico's security staff to hand over any information "that could help clarify what happened", it said.
The incident drew sharp condemnation from Spanish Prime Minister Pedro Sanchez, who on Tuesday criticised Atletico for not taking a stand against the racist chants from their supporters.
"I'm a big supporter of Atletico Madrid, so I was very sad," he told Politico on the sidelines of the UN General Assembly in New York, saying he had "expected a strong message from the club against this kind of behaviour".
Atletico later released a statement saying the abuse was "unacceptable" and that they would not stop until the culprits were found and banned.
"Atletico Madrid condemns the unacceptable chants that a minority of fans sung outside the stadium before the derby," it said.
"We cannot allow our fan base be related with this type of behaviour and question our values because of a minority who do not represent us."
La Liga said it would report the incident to the Spanish Football Federation's disciplinary committee.
The incident took place just days after Pedro Bravo, head of the Spanish Football Agents' Association (AEAF), sparked a furore by saying on television that Vinicius needed to stop "acting like a monkey" when celebrating goals.
The 22-year-old player is known for celebrating a goal with his trademark dance moves.
But Bravo's remarks touched a nerve in Brazil, where black players have been heckled with racist taunts by fans mimicking monkeys, with several big names saying his comment smacked of racism.
Bravo apologised, saying he was using a phrase that was not intended to have racist connotations. The phrase is commonly used in Spain to mean "playing the fool".