The FBI used the FBIs Network Investigative Technique (NIT) to deanonymize suspects while exploiting Tor network. The NIT allowed them to identify the IP address of Tor users. The FBI decided to use the server as a bait for online pedos, then the Bureau placed the NIT on the servers and used them to de-anonymize TOR users accessing the illegal content. The researchers were able to determine that if a TOR browser accessing the FBI controlled website had proper up-to-date controls configured the. NIT would not be able to reveal the true IP address.”]
Source: https://securityaffairs.co/wordpress/38213/cyber-crime/nit-fbi-deanonymize-tor.html