In 2013 the FBI agents seized TorMail, at the time the most popular dark web email services. The US law enforcement agency seized the TorMail database during in concomitants of the seizure of Freedom Hosting. The network investigative technique (NIT) was used to de-anonymize users on the Tor network. The use of the NIT was also confirmed earlier this year when according to court documents reviewed by Motherboard, the FBI had used it to identify the suspects.”]
Source: https://securityaffairs.co/wordpress/43865/cyber-crime/fbi-hacked-tormail.html