No details, though:
According to the complaint against him, Al-Azhari allegedly visited a dark web site that hosts “unofficial propaganda and photographs related to ISIS” multiple times on May 14, 2019. In virtue of being a dark web site—that is, one hosted on the Tor anonymity network—it should have been difficult for the site owner’s or a third party to determine the real IP address of any of the site’s visitors.
Yet, that’s exactly what the FBI did. It found Al-Azhari allegedly visited the site from an IP address associated with Al-Azhari’s grandmother’s house in Riverside, California. The FBI also found what specific pages Al-Azhari visited, including a section on donating Bitcoin; another focused on military operations conducted by ISIS fighters in Iraq, Syria, and Nigeria; and another page that provided links to material from ISIS’s media arm. Without the FBI deploying some form of surveillance technique, or Al-Azhari using another method to visit the site which exposed their IP address, this should not have been possible.
There are lots of ways to de-anonymize Tor users. Someone at the NSA gave a presentation on this ten years ago. (I wrote about it for the Guardian in 2013, an essay that reads so dated in light of what we’ve learned since then.) It’s unlikely that the FBI uses the same sorts of broad surveillance techniques that the NSA does, but it’s certainly possible that the NSA did the surveillance and passed the information to the FBI.