I must be missing something, I buy Bitcoin with a USD bank account, I send to another address and then this address sends to a known drug dealer. How do they establish the second address's identity?
I guess the investigators found out somehow, for example by looking at transactions on a computer already seized for realayed investigations. Also by sending bitcoin to the service themselves, the investigators could trace the flow of coins into addresses used by the dealers to spend their bitcoins. Then backwards from there to other users payments.