I read in this article that the FBI was able to crack the anonymity of TOR.

From what I heard, 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 (The article was 2 to 3 years old so it may not be that relevant now).

Do anyone have any insights on how "they think" the FBI did?