A man has been arrested on suspicion of a racially aggravated offence following Chelsea's League Cup win over Manchester United.
Pictures taken during Wednesday's fourth-round tie at Stamford Bridge appeared to show a fan making a "monkey" gesture.
Gavin Kirkham, 28, was bailed after being questioned at a west London police station on Monday.
United striker Danny Welbeck appeared to be the target of the alleged abuse.
The Metropolitan Police said Mr Kirkham was arrested after attending the police station by appointment.
He was released on bail until later in November.
Source outside game
In a separate development, the Professional Game Match Officials Ltd has said that referee Mark Clattenburg will not officiate in any matches this weekend.
Chelsea accused the referee of using an offensive racial term during the 3-2 Premier League defeat by Manchester United on 28 October.
Clattenburg maintains he is innocent and neither of his assistants, nor the fourth official, are backing Chelsea's allegations.
The Metropolitan Police also started an investigation into the allegations after receiving a complaint from a source outside the game.
Source: www.bbc.co.uk