The owner of an aerial photography business has been sentenced to 30 days in jail after a drone he was operating knocked a woman unconscious.
Paul Skinner was found guilty of reckless endangerment, by a judge at the Seattle Municipal Court.
It is believed to be the first time a drone pilot has been given a jail sentence.
The incident happened during Seattle's 2015 Pride Parade.
According to the police, the 18in (45cm) by 18in drone crashed into a building and fell into the crowd, injuring two people.
Judge Willie Gregory said he acknowledged that the incident was an accident but added that the pilot had "engaged in conduct that put people in danger of being injured".
Prosecutor Pete Holmes said the faulty operation of drones was a "serious public-safety issue that will only get worse" and more prosecutions could follow.
Ravi Vaidyanathan, a drone expert from Imperial College London, said he was "not aware of anything previously resulting in jail time".
He said it was inevitable more accidents would follow and called on the regulatory bodies that governed drone use to provide "a consistent set of guidelines on usage".
"In the US, there are rules for commercial use but different ones for hobbyists. We are in uncharted territory, but the guidelines have to be consistent."
He added the inconvenience to the public was also "non-trivial".
"Having a drone buzzing around does not add to most people's beach experience, for example, and we need to think about this too."
During the past fiscal year, more than 1,200 possible collisions between an aircraft and a drone were reported to the Federal Aviation Administration.
The FAA was unable to confirm any strikes, but it has reported several close calls, including a Lufthansa jet approaching Los Angeles that passed within 200ft (60m) of a drone.