Fulham will take the "strongest action possible" after full-back Cyrus Christie alleged that his sister was hit and racially abused by fans during Saturday's 1-0 defeat at Barnsley.
Christie said on Twitter after the game: "To the Fulham fan that decided to hit my sister at the game today and his wife who justified his action with racist remarks I hope you're proud of yourself and feel like a big man.
"You'll get what's coming to you. Regardless how you felt about the result your actions are shameful."
The Whites, relegated to the Championship last season, said in a statement: "Fulham Football Club is investigating an incident which took place in the away end at the Barnsley v Fulham fixture this afternoon.
"Should the perpetrators be identified, the club will be taking the strongest possible action."
In a statement, South Yorkshire Police said they were yet to receive a report of the incident.
"We are aware of claims circulating on social media," the statement said. "We are looking into the circumstances of the alleged incident and are in the process of contacting Fulham Football Club to progress the matter."
Republic of Ireland international Christie, 26, joined Fulham from Middlesbrough in January 2018 and has made 33 appearances for the club.
In November 2017 and April 2018 he shared racist abuse he had received on social media.