The air units had to retreat, because smaller UAVs are considered a hazard: if one hits a plane or a chopper, it could endanger not just the pilot, but also the people on the ground.

As US Forest Service's John Miller told NBC Los Angeles:

It can kill our firefighters in the air... They can strike one of these things and one of our aircraft could go down, killing the firefighters in the air. This is serious to us. It is a serious, not only life threat, not only to our firefighters in the air, but when we look at the vehicles that were overrun by fire, it was definitely a life-safety threat to the motorists on Interstate 15.

There are no confirmed injuries among the affected motorists -- they managed to run out of their vehicles before they went up in flames -- and we hope everybody really did get out on time. If someone did get injured or killed due to the delay, though, the drone owners would be in even bigger trouble. California Fire Captain Richard Cordova told The Hollywood Reporter that those owners would be held liable, as their machines prevented the firefighters from helping those who needed it.

This isn't the first time drones have hampered firefighting efforts, by the way: some UAVs were also spotted at another California wildfire in June. This incident dubbed the "North Fire" devoured 3,500 acres of land, around five houses and 20 of the 75 or so abandoned vehicles within four hours. Authorities are now looking for the people behind those five drones. While they likely didn't cause death or injury, Chief Marc Peebles of the county's fire department said the delay they created is definitely one of the reasons why the fire ended up spreading on the freeway.

#NorthFire (Update) Firefighting aircraft has been grounded due to drone activity in the area pic.twitter.com/aVkx5Ud2JU — SB County Fire (@SBCOUNTYFIRE) July 17, 2015

[Image credit: ASSOCIATED PRESS]