DJ Campbell has been rearrested in connection with allegations of spot-fixing.
The striker, who the Lions signed on loan from Blackburn Rovers in January, was one of six players initially arrested last December and bailed until April 8.
But the National Crime Agency has announced that those men are again in custody and being interviewed at police stations across the country in light of "new evidence".
A further seven footballers have also been held by the agency.
Millwall declined to comment.