JUNCTION CITY, Calif.—After a long day battling one of Northern California’s most stubborn wildfires, dozens of weary firefighters gathered in a remote wilderness clearing near the fire’s front lines to get a chopper ride back to camp.

Two veteran pilots flying a Sikorsky S-61N, a workhorse helicopter that can carry 16 passengers, had ferried out two groups and returned for another. The third group loaded up and lifted off, but then encountered a problem.

“They went forward a slight bit. Then the aircraft rapidly descended and hit the hillside,” said Andy Mills, chief of helicopter operations for Carson Helicopters Inc., which owned and operated the chopper. “Right now we don’t know why that happened.”

Two days after the helicopter crashed after taking off in the remote Shasta-Trinity National Forest, relatives, friends and co-workers of the nine people killed in the accident waited Thursday for clues that would help explain what went wrong.

A sheriff’s search term had not begun recovering bodies from the charred wreckage of the helicopter, which quickly burst into flames, because federal investigators needed to first assess the scene, authorities said. A National Transportation Safety Board team heading to the crash site hoped to recover a voice-data recorder from the downed aircraft, NTSB spokeswoman Kitty Higgins said.

The wreckage was still smoldering Thursday.

According to Carson officials, who described the crash as the company’s first firefighting accident in its 50-year history, there were no obvious warnings of danger Tuesday night.

“We’ve talked to pilots of our other two aircraft flying in the area,” Mills said. “So far it sounds to me like visibility was not an issue. It was not windy up on that ridge top.”

One pilot told a mechanic shortly before the fatal flight that the aircraft “was flying very well,” Mills said.

Even so, the helicopter plunged out of the sky just after takeoff, officials said, coming to rest on a steep outcropping 1,000 feet below where it left the ground. The helicopter was refueled just before the crash and burst into flames after it hit the hillside.

About 30 firefighters and other personnel in the clearing waiting for their own rides scrambled down toward the crash site in hopes of rescuing anyone who survived. According to Higgins, two survivors emerged in flames. Several witnesses reported that a third escaped without serious burns and returned to pull the fourth survivor from the wreckage.

The four survivors—three firefighters and a pilot—were flown to hospitals Tuesday night with severe injuries.

Authorities can confirm with “fair certainty” that all nine others—seven firefighters, a U.S. Forest Service employee and the helicopter’s pilot—died, Undersheriff Eric Palmer said.

Ten of the firefighters, including the three in the hospital, were employed by firefighting contractor Grayback Forestry, according to Kelli Matthews, a spokesman for the company.

The firefighters believed to have died in the crash were identified by Grayback as Shawn Blazer, 30; Scott Charleson, 25; Matthew Hammer, 23; Edrik Gomez, 19; Bryan Rich, 29; and David Steele, 19. All were from southern Oregon.

Grayback said it would not release the name of a seventh victim until it could notify family members.

Carson Helicopters identified the dead pilot as Roark Schwanenberg, 54, of Lostine, Ore.

Schwanenberg “lived and breathed” flying to fight fires, his wife, Christine Schwanenberg, said Thursday. “He felt responsible for making a difference in this world.”

The U.S. Forest Service did not immediately identify its employee, who was described as a flight inspector on the helicopter.

The firefighters had been working at the northern end of a fire burning on more than 27 square miles in the national forest, part of a larger complex of blazes that is mostly contained.