I read in [this article](http://www.ibtimes.co.uk/fbi-crack-tor-catch-1500-visitors-biggest-child-pornography-website-dark-web-1536417) that the FBI was able to crack the anonymity of Tor.

From what I heard and read, onion routing makes it almost impossible to de-anonymize a user.

The last time I heard anyone trying to crack Tor was the NSA and it did not succeed or at least they did not publicly advertise it (The article was 2 to 3 years old now and may not be that relevant anymore).

Do anyone have any insights on how the FBI might have done it?