Two members of a helicopter crew battling the Dome Fire in Yosemite National Park were killed in a crash on Sunday, the U.S Forest Service said.

The two pilots, who have not been publicly identified, were the only people on board and were conducting fire suppression operations in the national park, said Adrienne Freeman, a U.S. Forest Service spokesperson.

Both the NTSB and FAA said they are investigating.

The FAA confirmed the crash occurred around 3 p.m. Sunday and involved a Eurocopter AS32 helicopter.

A spokesperson for the NTSB said its personnel will be on scene “when fire conditions allow.”

The aviation company Fly Precision said the pilots were part of its team.

“It is with unimaginable sadness and broken hearts that we share the loss of two beloved members of our team,” the company said in a statement Sunday. “We ask for privacy and compassion for their families and our team as we grieve this tremendous loss.”

On Monday, the Dome Fire, which started in Mariposa, California, on September 15, was larger than 3,000 acres and only 15% contained, according to wildfire authorities.

The fire “continued to spread at a moderate rate of growth” and has swept further into the 2017 Empire Fire footprint, officials said.

The cause of the fire is under investigation, authorities said.

The NTSB spokesperson said a preliminary report on the helicopter crash will be released within the next 30 days.