NTSB investigating a helicopter collision that left 3 dead while battling Southern California fires

National Transportation Safety Board investigators are headed to the spot where three people were killed after two helicopters collided in midair Sunday while fighting a fire in Southern California, an agency spokesperson told CNN on Monday.

The helicopters were among six aircraft responding to the Broadway Fire, which began Sunday after a structure fire spread to nearby vegetation in Cabazon, a community in Riverside County some 90 miles east of Los Angeles, CalFire Southern Region Chief David Fulcher said Sunday night.

A Bell 407 helicopter and a Sikorsky S-64E helicopter collided in the air, the NTSB spokesperson said.

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

The three victims were identified by a Cal Fire official as Division Chief Josh Bischof, Captain Tim Rodriguez, and pilot Tony Sousa. The men are the first victims of California's 2023 fire season, according to data from Cal Fire.

"When these firefighters go to work every day they don't expect not to return home, and yesterday was a difficult day," Cal Fire Deputy Director of Communications Nick Schuler told CNN's Brianna Keilar on "CNN News Central."

"This is an extremely rare event, so it's extremely important that the investigation is methodical, but currently at this point, it remains under investigation," Schuler said of the crash's cause.

The Broadway Fire covered about three acres when firefighters responded around 6 p.m., according to a post on a joint Twitter account for CalFire and the Riverside County Fire Department. By shortly after 7 p.m., the "forward rate of spread" had been stopped, the agencies said.

A second four-acre fire ignited by the helicopter crash has since been extinguished, Fulcher said.

"I would like to express our deepest sympathies and sorrow to the families and co-workers of the personnel," Fulcher said. "This was a tragic loss for the community, the fire service community and CalFire and Riverside County Fire Department."

CNN's Melissa Alonso and Cherri Mossburg contributed to this report.