The authorities regularly withhold information that could assist defendants so it would be business as usual I'm sure. But yeah, that's one giant can of worms!
Theoretically, they are not allowed to do that. If it is evidence that would affect the outcome of the case, they are obligated to disclose it. While it seems like due process and the rule of law is being eroded daily, I really hope that the below case still applies...
http://en.wikipedia.org/wiki/Brady_v._Maryland