Everton and the Football Association are investigating after an object was thrown towards former Toffees player Ross Barkley during the Premier League win over Chelsea on Sunday.
The incident occurred in the seventh minute as Chelsea took a corner in front of Goodison Park's Gwladys Street End but the item did not hit anyone.
It was Barkley's first trip to Everton since a £15m move to Chelsea in 2018.
The FA have written to both clubs asking for their observations.
Everton told BBC Sport in a statement: "We are investigating the alleged incident and reviewing footage. If anyone is found responsible for this or other such incidents, the club will take robust action."
The incident came two days after the FA, the Premier League and EFL issued a joint statement on Friday asking supporters "to carefully consider their behaviour, not only at this weekend's Premier League, EFL and FA Cup matches, but at all games in the future".
The advice was issued after three incidents the previous weekend when Aston Villa's Jack Grealish was attacked by a Birmingham City fan, while Manchester United's Chris Smalling and Rangers skipper James Tavernier were also confronted by spectators.
Barkley was booed throughout the game at Goodison before he was substituted after 65 minutes.
Everton won 2-0 thanks to goals from Richarlison and Gylfi Sigurdsson, after which Barkley was taunted by fans chanting "Barkley, Barkley, what's the score?".