The Federal Aviation Administration is investigating reports of a drone aircraft that was seen flying over the 4/20 marijuana rally in Denver on Sunday.
FAA spokesman Allen Kenitzer said Tuesday people need permission to fly aircraft over crowded areas.
According to KMGH-TV, smaller drones are sometimes considered model aircraft, which don't require authorization but do have to follow FAA guidelines. Those guidelines say hobby-aircraft flyers must keep their aircraft below 400 feet above ground level and away from airports and air traffic.
Denver police say they found two unidentified men flying a drone over the rally in Denver's Civic Center Park. The men were asked to leave the top of the McNichols Civic Center Building because they didn't have the proper permission.