Our sister network ESPN is reporting that Brown is facing a suspension over a confrontation Wednesday with General manager Mike Mayock.
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Reports say the incident involved Brown's posting of a letter from Mayock on social media, informing the receiver that he had been fined.
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Brown was not at practice on Thursday.
Neither Mayock nor Coach Jon Gruden is saying why.
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If suspended, the Raiders could void $29 million in guarantees in Brown's contract he signed earlier this year.
It's been a tumultuous pre-season for the 31-year old, from a case of frostbitten feet, to a confrontation with the NFL over his helmet.
ESPN has contributed to this report.