(CBS SF) — As thousands of revelers start making the trek home from the Burning Man festival in Northern Nevada, newly released documents reveal the FBI spied on the event a few years ago.

Documents obtained by the journalist Inkoo Kang, and posted to the website MuckRock, reveal that the event has been under FBI surveillance since at least 2010. That’s when the agency concluded that Burning Man is “considered a cultural and artisan event, which promote (sic) free expression by the participants.”

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In one document from 2010 agents conclude that the “greatest known threat in this event is crowd control issues and use of illegal drugs,” but in a heavily-redacted report produced less than a month later, the agency’s Special Events Management unit seems to note that the event is used to practice intelligence collection and anti-terrorism security.

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The FBI declined to comment on the Burning Man files when contacted by 48 Hours Crimesider.