Some sick people have exchanged or downloaded porno with kids and they were so many (bored with normal porn or just good sex ?) that they overcharged a server of a German ISP who found all that traffic on its fileserver a bit too much to be normal and had a look at what was happening.
They informed the police and the police came (with the press in their footsteps). The articles were published all over the world and the world knew that
a. there were 12.000 people involved in more than 70 countries
b. the police was still investigating all the logs and material
c. the police will still have to visit each of them for interrogation and probably the confiscation of their computers. In Belgium they will do this next week they say in the newspapers.
So, but how many will have changed address, ISP, computer or have thrown away everything they have that looks like computerdata ? And the interrogation will go a bit like this probably :
Oh, Why did I throw my computer away? It was so infected with viruses and hacked all the time that I just bought a new one and destroyed this one completely. Oh and all that data ? I threw it also away because there would probably be so many viruses on it that I would just infect my new computer and I didn't want to take any risk. You say that I went to a German server ? But I don't speak german ? No, I only read the newspapers on the internet, look at my internetuse now. It must have been viruses or what do they call them, botnets and trojans that kind of stuff....
Ok you can have other evidence and logs but it still will have to stand up in court against any reasonable doubt. And how many prosecutors would take the risk of having one weak case that could blow all even the stronger cases off the table because there is not enough inrefutable evidence ?