In NFL games I'm pretty sure there are enough officials to cover both sides of the line on a try. He called it as a dead ball foul, play was stopped.
From what I could tell the umpire was the only one that threw the flag. As an umpire, even on a try, I would never call an offsides. Encroachment maybe. But really the umpire is just supposed to be used in a support role for that call. The just don't have the angle to call that.
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