Raiders WR Antonio Brown not practicing, general manager Mike Mayock says

    Antonio Brown warms up before the NFL preseason game against the Arizona Cardinals at State Farm Stadium on August 15.

    (CNN)Antonio Brown finally has a helmet, but it appears his troubles are not over.

    The Oakland Raiders wide receiver was not at the team facility on Thursday, general manager Mike Mayock told reporters. The team did not say why or if he was being disciplined.
    "Antonio Brown is not in the building today," Mayock said. "He won't be practicing. I don't have any more information for you right now. And when I have some, and it becomes appropriate, you guys will all get it, I promise you. But that's it for today."
      The Raiders' injury report shows that Brown did not participate in practice Thursday with the description as: "Not injury related - conduct."
      Brown took to Instagram Wednesday, posting a letter purportedly from Mayock informing him he has been fined close to $54,000 by the team for missing time during preseason training camp.
      According to NFL Network's Ian Rapoport, who cited sources familiar with the situation, Mayock was frustrated Brown posted the letter.
      Rapoport also reported that the two got into a heated exchange on the practice field, which was witnessed by other players. According to Rapoport, things did not get physical.
      ESPN's Josina Anderson also reported that Brown and Mayock were in an argument during Wednesday's practice. ESPN's Adam Schefter was first to report that the confrontation stemmed from Brown's Instagram post.
      CNN has reached out to representation for Brown but did not immediately hear back.
      Raiders head coach Jon Gruden said he would not get into Brown's situation.
      "I'm emotional about it, hope you understand why," Gruden said. "I think a lot of this guy, I think Antonio is a great receiver, and I think deep down, I think he's a really good guy. So I'm frustrated. I'm not going to say any more about it. Hope it all works out. But I don't have anything official to say about anything else until I get all the facts, and that's what I'm going to do."
      Brown, who is in his 10th NFL season, missed time during training camp while he recovered from frostbite on his feet from doing cryotherapy -- and while he was appealing to the NFL to use the same helmet he has used his entire career.
      The problem: His preferred helmet was one of the older models that had been banned by the NFL and the NFL Players Association beginning this season to improve player safety.
      He filed two grievances, losing both.
        On Wednesday, Brown announced he will use a Xenith Shadow helmet this season.
        The Raiders open the regular season in Oakland against the Denver Broncos on Monday.