Wolverhampton Wanderers told fans they faced prosecution after objects were thrown at Manchester City players during the teams’ Premier League match.
After City celebrated Raheem Sterling’s opening goal, an announcement told supporters “numerous instances of objects being thrown” had been seen.
One item looked like a metal hip flask.
BBC Radio 5 Live’s football correspondent John Murray, at Molineux for Friday’s game, said it “could have caused serious damage”.
Referee Martin Atkinson gave the object – thrown from the Wolves end behind keeper Rui Patricio’s goal – to fourth official Andre Marriner, who informed security.
The incident happened after City had taken a 1-0 lead in an action-packed first half.
Manchester City keeper Ederson was sent off and Sterling then had a penalty saved twice – a retake having been ordered after the video assistant referee punished encroachment from Wolves captain Conor Coady – but the City forward scored from the rebound on the second spot-kick.