Six people have been arrested over a banner, which reportedly attacked a footballer who was racially abused by a Millwall fan, police said.
The banner was unfurled in a Millwall section of the Lion's New Den ground during their 2-1 win against Derby County.
Bolton striker Marvin Sordell, was abused by fans while warming up for his side's game at the East London club's stadium on 6 October.
A 13-year-old boy was banned from the ground.
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The England footballer has accused the FA of actions "bordering on blackmail", in relation to complaints about bullying.
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The ex-England women's coach was found to have made "ill-judged attempts at humour" but his comments were not deemed racist.