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DENVER -- The Federal Aviation Administration has launched an official investigation into what may have been an unauthorized drone flying above Denver's 4/20 rally at Civic Center Park on Sunday.

Allen Kenitzer, a spokesperson for the FAA Northwest Mountain and Alaska Regions, confirmed the investigation had been launched as of Tuesday, saying even unmanned aircraft need some level of FAA authorization before utilizing U.S. airspace.

An FAA Advisory Circular states that any hobbyist flying an aircraft higher than 400 feet requires some measure of FAA authorization. Flying any sort of unmanned aircraft lower than that mark is subject to FAA regulations, but doesn't require the same sort of authorization.

A key additional point in the investigation will likely be whether the pilots collected any commercial footage with the drone. If that occurred, the pilot or pilots would be in violation of a specific FAA regulation that prohibits the use of unmanned aircraft for commercial purposes.

It's important to the note that the FAA regulation in question identifies posting footage to YouTube as a commercial venture as well, according to Jeff Cozart, the manager of the Unmanned Aircraft System Program at CU-Denver.

"(This regulation) is primarily intended to restrict individuals from receiving payments or other financial benefit for services around UAV operations," Cozart said. "It allows for a distinction between model aircraft operators -- also known as hobbyists -- and those flying unmanned aircraft systems for commercial purposes."

A company that asked to remain unnamed because of the sensitivity of the information told FOX31 Denver it had a drone flying over the rally on Sunday, and that there were actually four drones in the air during various times of the event.

It had not yet been confirmed as of Wednesday whether any of those drones was flying above 400 feet, or if any drone-produced footage had been used for commercial purposes.

"All I can say about this (drone) incident is that it is under investigation," Kenitzer wrote in an email.

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