Firefighters working to contain a wildfire in the San Gabriel Mountains of Southern California were temporarily thwarted this weekend when pilots for the Monrovia Fire Department (MFD) spotted a few private drones in their path. For low-flying fire-fighting planes carrying fire retardant and smoke jumpers, an errant drone could mean life or death for the pilot and any crew. As such, the fire department decided to temporarily ground all aircraft on Saturday morning.

That decision can be a frustrating one for firefighters and residents of a fire-affected area, because there’s always the potential that the fire could burn out of control without aircraft flying in firefighters and equipment. The Monrovia Fire Department acknowledged this situation in a post this weekend

“It is vitally important to note… fire officials cannot deploy firefighting aircraft when private individuals are flying drones in the fire response locations," the MFD wrote. "Fortunately for us here locally, the fire was more fully contained when we had to suspend air operations yesterday, however, these types of disruptions are extremely dangerous to firefighting personnel and can cause severe disruptions to the response effort.”

According to the National Wildfire Coordinating Group (NWCG), the San Gabriel fire is currently 72 percent contained. Nevertheless, the Federal Aviation Administration had issued a Temporary Flight Restriction (TFR) over the area, so the pilot or pilots of the drones could face fines if they’re discovered.

Amateur drones grounding firefighting aircraft is not a new problem. Last year, the US Forest Service started a campaign to try to curb the practice. State lawmakers tried to increase fines for those who are discovered flying a drone too close to fire-fighting operations, although California Governor Jerry Brown vetoed that bill.

This latest California incident is hardly the first in the country this year. Firefighters in Arizona and Utah have also had their jobs interrupted by amateur drone pilots. According to US Forest Service public affairs spokeswoman Jennifer Jones speaking to The Arizona Republic, nine drones sightings have been reported in areas where firefighters were working. “At least three of which forced fire managers to ground their air crew,” Jones said. That’s compared to 2015, when 21 drone sightings were reported to the US Fire Service and nine of those forced the grounding of fire-fighting aircraft.

The Arizona Republic also reports that during a recent fire in Utah, a drone came “within feet” of a firefighting helicopter. This prompted Utah Governor Gary Herbert to comment that some resident evacuations might not have been necessary if drones hadn’t prevented Utah firefighters from flying in the area.

According to the Associated Press, the Interior Department is currently “working with drone makers and mapping companies to create a system that uses smartphone apps already on the market to quickly alert drone fliers to temporary flight restrictions at wildfires.” Such a setup matches many of the solutions Ars saw from app-makers and major companies like Amazon at the recent AUVSI drone conference.

Ars has contacted the Interior Department for comment and will update this post when we receive a response. Until then, official recourse could come if a drone gets in an accident and is recovered, as the FAA recently started requiring amateur drone operators to register their unmanned aerial vehicles.