I find it hard to imagine that it would be exceedingly difficult for Interpol or even the FBI to trace any hidden service. Fact of the matter is, no matter how much some uninformed people may argue against, that hidden services are not very anonymous. There have been attacks carried out on the live Tor network that have traced hidden services, this was done in 2006. The attack is simply opening an arbitrary number of circuits to the hidden service, this causes the hidden service to open new circuits as well. Then you send the hidden service a watermarked stream and look for it at all of your Tor relays. You can massively reduce the amount of time it takes to trace a hidden service with this attack, back in 2006 they were finding them in a matter of minutes with minimal resources. After that research was published, the Tor developers tried to counter the problem by adding entry guards. Now Tor clients and hidden services select three Tor nodes with the entry flag and always enter the network through one of these nodes. The nodes used for entry guards are selected before your very first Tor circuit is formed, and new guards are selected every month to two months. This defense prevents the attacker from tracing directly up to the hidden service, because if they do not own an entry guard now they can only actively trace up to the entry guards until one of their entry guards are selected. I am not the biggest fan of this situation, it essentially means that with very little effort an even very weak attacker can trace a hidden service up to three points that have a direct link with it. If these points are in the USA or a cooperating country, there is nothing stopping Interpol or the FBI from passively monitoring them. If they passively monitor the entry guard, at its ISP for example, they will then be able to fully deanonymize the hidden service. If none of the entry guards are in places they have any power in, they can keep waiting until it rotates to a set of entry guards they own or can passively monitor. After locating a hidden service it would be counterproductive for them to immediately take it down or announce their bust. Rather they would monitor traffic to it. Now they have met half of the requirements of a timing attack, they can see traffic arrive at its destination. This means that they can now deanonymize anyone who uses one of their entry guards to access the hidden service. As I said before, the Tor program selects three entry guards and it rotates them every month to two months. The probability that you will select a given entry guard also correlates positively with the amount of bandwidth being offered, and with several other factors as well. If they get a couple of high bandwidth Tor nodes, forty thousand dollars worth of servers and bandwidth perhaps, they can probably get several target Tor users using their entry node in any given month. At this point they will have broken the anonymity provided by Tor to the hidden service and to those particular clients. However there would still be a lot of people who they haven't gotten yet, and they can not pick their victim, it is like throwing out a fishing net and seeing what you drag up not like putting a deer in the sights of your rifle. The number of X people they can deanonymize in Y time will depend on a few things, for one the total size of the target group (the more people they are interested in the more likely they will get some of them), and especially the total percentage of bandwidth that their entry guard nodes handle for Tor.