Three members of a firefighting helicopter crew were killed after their helicopter collided with another in midair and crashed while battling wildfires in southern California on Sunday evening, officials said.

The cause of the deadly accident near Cabazon, about 90 miles east of downtown Los Angeles, was not immediately clear, the chief of Cal Fire’s southern region, David Fulcher, told a news conference early Monday.

CBS News Los Angeles described a “fiery mid-air collision”.

Both helicopters were responding to a grassland fire when they collided Sunday about 6pm. The pilot and two other crew members of the coordinating chopper – an “observer helicopter” – were killed. The other helicopter, a water-dropping one with two crew members aboard, was able to land safely.

“The first helicopter was able to land safely nearby. Unfortunately, the second helicopter crashed, and tragically all three members perished,” Fulcher said in a news conference.

Fulcher added: “I would like to express our deepest sympathy to the family and co-workers of the personnel. This was a tragic loss.”

The victims killed in the crash included a contracted pilot, a CalFire division chief and a CalFire captain.

Investigators with the National Transportation Safety Board are assessing the crash and the circumstances surrounding it.

Sunday’s deaths are the first of California’s 2023 fire season, according to data from CalFire. They come two weeks after a water bomber crashed while fighting forest fires on the southern Greek island of Evia, killing the two pilots.

California’s fire season has gotten off to a slow start after wet weather improved the devastating drought levels that have plagued the region and delayed the onset of fire weather conditions.

But experts have warned that the reprieve would not be permanent. A historic heatwave began blasting the lower south-west US in late June, stretching from Texas across New Mexico and Arizona and into California’s desert.

Environmental analysts have also cautioned that as climate warming gets worse and temperatures continue to rise, fires are likely to increase.