Police carried out dawn raids to arrested three men in connection with violence at an FA Cup match between Millwall and Wigan.
The men, aged 21, 22 and 27, were arrested at addresses in London by officers from Operation Arrowtip, which targets football-related crime.
So far 23 people have been arrested in relation to violence at the semi-final at London's Wembley Stadium on 13th April, during which Millwall fans attacked police and fought among themselves inside and outside the ground.
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."