What was the predators of an trex?

Given it´s size it´s probable that T-rex was not a great runner, therefore it´s likely that it could have been an ambush predator. There is also a suggestion that it could have been a huge scavanger. Although is likely that T-rex wouldn´t pass the opportunity to steal a meal from a smaller predator, it is hard to imagine that such a big creature would only rely on what it could find laying dead and half eaten.