The FBI made security researchers hand over data that helped identify people suspected of using the Tor anonymising network, a court case has revealed.
It used a court order to access data gathered by two researchers during a 2014 Carnegie Mellon University project funded by the US Department of Defense.
The home IP address of American Brian Farrell was among those obtained by the FBI via its analysis of the CMU data.
He denies helping to run the Silk Road 2.0 website, which sold drugs via Tor.
Tor, also known as The Onion Router, lets people browse the web anonymously by bouncing data through a series of routers that encrypt it at each step.
This has also given rise to a number of hidden sites, including many markets selling drugs and other illegal items and services, that sit on the Tor network.
Once identified, the researchers said they would give a talk at the Black Hat security conference about the methods they had used to "de-anonymise hundreds of thousands Tor clients and thousands of hidden services".
The talk never went ahead - but later in 2014, the FBI mounted a series of raids on people suspected of running sites sitting on Tor.
The revelation comes as the FBI is embroiled in a row with Apple over a request to unlock an iPhone used by Syed Rizwan Farook who, with his wife, killed 14 people in California last year.
The trial against Mr Farrell is due to start in April.