Catcher's Interference

[harmbu]

This happened in the Junior High game under FED rules. After a pitch crosses the plate, the batter swings and hits the catcher's mit. The only thing is that the catcher had already caught the ball when the contact was made with the mit. Catcher's interference was not called and the umpire explained that he cannot call interference when the ball is already in the glove. I have no reason to disbelieve him, but I cannot find it anywhere in the rule book or case book.

Anybody have any answers with rule references?

Thanks