Millwall fans took to the pitch after Harris scored against Leeds
The Football Association will investigate the crowd trouble which marred Millwall's play-off semi-final first leg against Leeds on Saturday.
A number of fans entered the field at the Den after Neil Harris scored for Millwall to give them a 1-0 victory.
Leeds goalkeeper Casper Ankergren was subjected to taunts by some Millwall supporters following the goal.
The Dane said he also had missiles thrown at him and one man was arrested on suspicion of assault.
He told BBC Sport: "There was something thrown at me, I didn't see what it was as I turned my back on it.
"It's very bad for football and Millwall should be punished."
The FA has confirmed it will make enquiries and gather information from all concerned parties.
Millwall have confirmed that any fans arrested will be banned from all their games for the foreseeable future and that Ankergren has received apologies from the club.
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