Tottenham say their initial findings following the alleged racist abuse of Antonio Rudiger are "inconclusive" - but a Chelsea fan has been arrested for allegedly abusing Son Heung-min.
Play was stopped during Chelsea's win at Spurs on Sunday after Chelsea's Rudiger said he heard monkey noises.
Spurs said they are "exhaustively investigating" the incident.
Meanwhile, police arrested a Chelsea fan for a racially aggravated public order offence against Spurs' Son.
A total of six arrests were made as part of the Metropolitan Police operation at the fixture but none were linked to the incident involving Rudiger.
"We have engaged lip readers to study the footage and contacted Chelsea for further information from their players," Spurs said in a statement. "The police will be reviewing our evidence alongside us."
The club added: "Please be assured we shall be exhaustively investigating this matter."
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Spurs said they are able to "track every fan" using cameras at the Tottenham Hotspur Stadium and that any supporter found to be guilty of racism will "receive a lifetime ban".
"This club has a proud track record of anti-racism work across all our communities and we are determined to ensure that we conduct a thorough investigation," the club added.
"At this time however we should point out that our findings are inconclusive and would ask that comment is reserved until the facts are established."
In a separate statement, the Premier League said it would support both clubs "in their pursuit of any perpetrators and call for appropriate action to be taken by the authorities and the clubs".
'Repeated announcements created misconception'
Chelsea captain Cesar Azpilicueta told referee Anthony Taylor of Rudiger's complaint during the second half of Sunday's fixture.
Three announcements over the public address system warned that "racist behaviour is interfering with the game" between the incident and full-time.
The Uefa protocol says if a message over the public address system does not stop the racist abuse in a stadium, a second announcement should follow and the temporary suspension of play should be enforced.
If discriminatory behaviour continues, authorities can decide to abandon the fixture.
Spurs said the fact they repeated the announcement created a "misconception" that the issue was ongoing in their stadium.
The club added: "In respect of protocols - when the incident was conveyed to the referee Anthony Taylor, he took the decision to call for the implementation of Stage 1 of the Uefa protocol - rather than the Premier League protocol - and asked for an announcement to be made, as well as requesting a further announcement which created a misconception that any issue was ongoing.
"The Premier League protocol differs from Uefa protocol in that it does not call for an announcement rather that the individual(s) be dealt with by the Safety Team in the first instance.
"We have asked that the Premier League clarifies the position regarding the use of these protocols to all stakeholders going forward."
In the aftermath of the incident, the Professional Footballers' Association called for a government inquiry.
The government has not ruled out taking "further steps if required".
On Monday, a host of Premier League managers were asked about the issue, with Newcastle's Steve Bruce stating he was "sickened and saddened by it", while Manchester City's Pep Guardiola said it will take "a lot of time" to "eradicate" the issue.