In 2015, the FBI seized a Tor-hidden child-porn website known as Playpen and allowed it to run for 13 days so that the FBI could deploy malware in order to identify and prosecute the website’s users. That malware, known in FBI-speak as a « network investigative technique, » was authorized by a federal court in Virginia in February 2015.

Source : After FBI briefly ran Tor-hidden child-porn site, investigations went global | Ars Technica