The FA has told ESPN it will investigate anti-Semitic chanting by West Ham fans during the club's 3-1 defeat at Tottenham on Sunday.
Throughout the 90 minutes, sections of the visiting support at White Hart Lane aimed chants that were either anti-Semitic or goaded their home counterparts about violent attacks on some Spurs fans before the club's Europa League game against Lazio in Rome last week.
They made hissing sounds early in the first half - a reference to the gassing of Jews in concentration camps during the Second World War - and chants of "Adolf Hitler's coming for you" were heard.
The FA has vowed to clamp down on racism and prejudice in football, and is likely to attempt to reach a swift verdict in this case.
On Monday afternoon, it released a statement which said: "The FA can confirm it has begun investigating reports of abusive chanting at the Tottenham Hotspur FC versus West Ham United FC fixture.
"The FA governance department has contacted both clubs and will continue to make inquiries into the matter in the coming days.
"There is no place for anti-Semitism or any form of discrimination in football. The FA is committed to working with the clubs, leagues, fans groups, police, CPS and community stakeholders to play our part in addressing this unacceptable behaviour."
In a statement, West Ham said they would be helping with investigations into what had happened: "West Ham United will take the strongest possible action against any of their supporters, including enforcing life bans from the club, found guilty of behaviour which is categorically not condoned by West Ham United.
"During the 46 games in the Championship last season, West Ham United had zero arrests for racism or violence, so while we are surprised to see such reports we will examine any available evidence of such conduct thoroughly and take the appropriate action."
As second statement from the Hammers said: "Tottenham Hotspur have confirmed that five supporters were arrested during Sunday's match at White Hart Lane outside of the stadium for unrelated incidents and two more were cautioned for racially aggravated gesturing inside the ground.
"One of those fans has since been identified as a season ticket holder and has, this afternoon, been sent a letter containing a banning order from the club. Any other individuals identified can expect a similar swift and robust response."
The FA said it had "noted the statement issued by West Ham United FC, and encourages clubs to identify and ban for life any individuals involved in incidents of abusive chanting".
The anti-racism group Kick It Out said it was examining the reports of abusive chanting. Chairman Lord Herman Ouseley said: "Kick It Out is following up with both clubs after reports circulated concerning unacceptable chanting at White Hart Lane, and is awaiting reports with further detail."
The Community Security Trust (CST), a charity that monitors anti-Semitism, said it had received complaints following the match, including many from people who were at White Hart Lane.
CST spokesman Mark Gardner said: "The days of English football crowds making mass monkey noises are thankfully gone, but mass anti-Semitic chanting about Hitler and gassing was clearly heard yesterday from a loud section of West Ham fans.
"We have heard such abuse against Spurs before, and it risks seriously compromising the work against racism at all levels of the game."
He said UEFA needed to "take urgent action regarding Lazio and West Ham", adding: "Fans who indulge in racist or anti-Semitic behaviour should be arrested, charged and banned.
"We cannot have the football family ignoring, and therefore encouraging, mass Nazi chanting."