[21]

Not impossible, just hard.

And the cops can go and track each individual person from that pool if they really care. Even if we are talking about thousands.

Remember the story from a few days ago where to track a possible spy they went through all glasses prescriptions from a city.

[lossolo]

It's different beast. It's almost impossible if done right. How would you track this person? You only see end transactions from those addresses which are not mixed with coins of attackers. You would need to check EVERY possible place where bitcoin exchange happen and there hundreds in hundreds of countries in blind to check if bitcoin address x was used there. Then some countries maybe even will not give you any information because electronic currency doesn't exist in their law and it's not a felony to use mixing service etc. etc. That's why they use bitcoin in the first place for 99% of criminal activities in Internet.