West Ham are looking into circumstances around a youth team coach who attended a march organised by the Democratic Football Lads Alliance.
Mark Phillips, who coaches West Ham's U18s, posted on Twitter about attending the DFLA's march in London last Saturday.
He went on to defend the organisation, which has been accused of having links with the Far Right, in further tweets.
Football's anti-racism organisation, Kick It Out, was made aware of Phillips' involvement in the march and subsequently reported the complaint to West Ham.
The demonstration descended into violence when anti-racist protesters attempted to block the DFLA's route, although there is no suggestion Phillips was involved in the disorder.
Phillips, in response to a man who said he was at the march, said on Twitter: "I was too. The football fans were sticking to the agreed route and Antifa chose to confront the march. They had their face covered up. We didn't. Ask yourself honestly who instigated the trouble??"
He added: "The police made a mistake by walking the peaceful football fans march into the aggressive Antifa group and not the other way round. Ask the police on duty for the truth.
"Who said they were far right? Not true. Football fans yes. Far right. Fake news. Admittedly there was a lot of ex-football hooligans there.
"But their behaviour was non aggressive until ambushed by people hell bent on causing trouble. No matter how people look it's their behaviour that counts. Ask your ex-colleagues the truth.
"The football fans were sticking to the agreed route and Antifa chose to confront the march. They had there face covered up. We didn't. Ask yourself honestly who instigated the trouble?? I can't comment I got the tube home from Embankment station and did not enter Parliament square. I only comment on things I actually witnessed."
A Kick It Out statement read: "We can confirm we've received a report about a West Ham coach claiming to have attended the DFLA rally in London at the weekend. We have passed it on to the club and are liaising with them about the issue".
A West Ham statement read: "West Ham United is an inclusive football club. Regardless of gender, age, race, ability, religion or sexual orientation, every supporter within our diverse fanbase is warmly welcomed at London Stadium, free to enjoy watching their team play football without fear, discrimination or abuse.
"We have a zero tolerance policy to any form of violent or abusive behaviour. We continue to protect and cherish those values and we remain committed to ensuring that every single member of the West Ham family feels safe, respected and included."