An air tanker dropping retardant on a remote wildfire along the Utah-Nevada line crashed Sunday afternoon, killing both crew members, authorities said.

The pilots were dropping retardant on the 5,000-acre White Rock Fire, which began burning Friday night after a lightning strike in eastern Nevada.

The fire spread across the Utah line Saturday night, but most of the blaze remained in Nevada, about 150 miles northeast of Las Vegas.

The cause of the 1:45 p.m. MDT crash was unknown, Bureau of Land Management officials said.

A helicopter crew saw the crash and told ground crews that “it didn’t look good,” Iron County sheriff’s Detective Sgt. Jody Edwards told the Salt Lake Tribune.

BLM ground crews and helicopter crew members worked for a time to hold the fire back from the wreckage. Sheriff’s deputies drove and hiked for more than an hour to reach the site and confirm that the pilots had died, Edwards said.

The fire later overwhelmed the crash site, according to Edwards.

The pilots were flying a P-2V air tanker that is owned by Neptune Aviation Services of Missoula, Mont.

The victims’ names weren’t immediately released.