The FBI is the primary party appearing to be in this investigation, and to the best of my knowledge are both of the US (govt), and are federal. (I'm not a US person so please correct me if I'm wrong.)
The article mentions the FBI investigation but is not about that. This article is about a pressure campaign, the letter Web Abuse Association Defense sent to adguard making threats under french law and adguard's investigation in response to that letter.
Which was what my parent comment referred to. Comments replying to mine act like it's unreasonable for me to talk about US federal agents in the context of this subject, when those are what the article mentions specifically, many times.