Police have said they are investigating after coins were thrown by West Ham fans during the Hammers' 2-0 defeat to Chelsea at Stamford Bridge.
The coins were hurled as Blues midfielder Frank Lampard, who joined Chelsea from West Ham in 2001, celebrated his 200th goal for the club with substitute John Terry and team-mates near the travelling supporters.
A spokesman for the Metropolitan police said no arrests had so far been made, but added: "We are investigating instances of coin-throwing at Chelsea verses West Ham on Sunday March 17. Inquiries continue."
Chelsea and West Ham will work together to investigate the incident, with CCTV footage being studied in an attempt to identify the targets.
The Hammers could be punished by the FA over what happened, and released a statement in which they said anyone found to have thrown coins faced the possibility of a ban.
"Any individual found to have acted in an inappropriate way will have to face the consequences of their actions, including the possibility of a ban from attending future matches,'' the statement said.
Information from the Press Association was used in this report.