His downfall you might say was in 1501, when he sheltered Edmund de la Pole, Earl of Suffolk and let him escape to France.
Tyrell never confessed to harming the Princes, his name wasn't mentioned in reference to their disappearance until ten years after his death. The most famous account, of course, was written by Thomas More in his History of Richard III, he states
The only images available to illustrate this post are of Tyrell doing away with the princes and that won't do! Instead, I've chosen to place an image of Thomas More writing his lies, and ensuring that Tyrell is seen as a villain for eternity.