http://www.bbc.co.uk/news/technology-39180204
The FBI and other agencies always use either paedophiles or terrorists as the justification for having mass surveillance and snooping powers. Now they might have caught one using those powers, they are dropping the case against him because they would be forced to reveal the details of the exploit they used, and this exploit would inevitably end up being fixed to prevent further snooping. Which begs the question, why do they need these powers if they aren't going to use them against the people they said they needed it for?