Last week it emerged that suspicious betting patterns relating to a booking picked up by an Arsenal player this season had been reported to the FA.
The Times then reported that it was Granit Xhaka's caution, the only one handed out to a visiting player by referee Andre Marriner on December 18 at Elland Road, that is at the centre of the issue.
Xhaka, who escaped punishment earlier in the game for a rash challenge on Raphinha, was booked in the 86th minute for time wasting.
When asked to confirm if that yellow card is at the heart of an investigation, an FA spokesperson told the YEP: "The FA is aware of the matter in question and is looking into it.”
The YEP understands a formal investigation is not yet underway, however.
According to The Times, FA sources have made clear there is no suggestion of any wrongdoing by Xhaka or other Arsenal players.
A sum of over £50,000 was reportedly placed on the yellow card before it was shown, leading bookmakers to involve the FA.