Honestly the FBI could put all Tor relays in the USA under constant passive surveillance. Trap and trace / pen register does *not* require a warrant, only good faith that it will aid in a criminal investigation. The ISP level technology to passively monitor Tor relays has been in the hands of the FBI since at least Carnivore in the late 90s. And it would technically be legal for the FBI to monitor all Tor relays in the USA. Many people think that NSA is the agency doing such attacks though, and not FBI. This case actually makes it pretty apparent that the FBI is not doing such surveillance as they would have almost certainly been able to identify these guys a lot faster if they had been. But it should be kept in mind that nothing is (apparently, please correct me if I am wrong....) preventing them from doing this, on a legal or technological level.