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SAN FRANCISCO (AP) — A medical transport plane broke up in flight last month and crashed into a densely forested California mountain range, killing all four people aboard, federal investigators have determined.

The National Transportation Safety Board’s preliminary findings emerged this week from interviews with witnesses, an investigation at the crash scene and data collection. No cause for the crash has been determined and a final report could take up to a year to complete.

The Piper PA31T was carrying a flight nurse, transport medic and patient from Crescent City, near the Oregon border, to Oakland on July 29 when the pilot reported smoke in the cockpit and declared an emergency.

The pilot planned to return to Crescent City but the plane vanished from radar 5 miles north of the Arcata-Eureka Airport.

The report says both engines separated from their firewalls, but it was not immediately clear if that happened in flight or on impact.

A woman camping nearby reported she heard an airplane circling overhead before it eventually flew to the west, which was followed by 15 seconds of silence. When she looked out of her tent, she saw a large, dome-shaped flash, followed by another flash and a loud rumble, according to the report.

“Unfortunately, this is the worst type of accident. This was a life-saving mission that was being undertaken that turned tragic,” said Jim Hall, a lawyer and safety advocate who was the chairman of the NTSB during the Clinton administration.

“I have long advocated that any of the aircraft that are performing these missions have two pilots because the pilots are multitasking carrying a surgery unit in the air and there is a lot of additional stress in regard to the flight mission,” he said.

The crash comes as the Federal Aviation Administration continues its efforts to improve the safety of the aircraft known as air ambulances. It began that effort after a series of deadly crashes. In 2008 alone there were five accidents that killed 21 people.

On Feb. 20, 2014, the FAA issued new rules requiring air ambulances to have stricter flight rules and procedures, more training and additional on-board safety equipment.

In the July 29 crash, rescue teams led by the Humboldt County Sheriff’s Department found the wreckage on land owned by a private timber company in the county about 280 miles north of San Francisco.

The plane was part of Cal-Ore Life Flight, which transports patients throughout Northern California and Oregon. Killed were pilot Larry Mills, 54; nurse Deborah Kroon, 49; paramedic Michelle Tarwater, 30; and patient April Rodriquez, 35. They were all from Crescent City.

Meanwhile, the NTSB also has released a preliminary report on the July 27 crash of a small private plane in the Northern California town of Columbia that killed the four family members aboard.

The report says witnesses reported seeing the 1958 Cessna 310B taxi onto a runway for departure, climb about 40 feet and drift to the left. The plane then crashed to the ground and burst into flames.

No cause for the crash has been determined.

Those killed were the pilot, Daniel Kruetzfeldt; his wife, Kristen Kruetzfeldt; his mother, Mary Chandler; and his stepfather, Claude Chandler, all from Sonora, California.

The Columbia Airport in Tuolumne County is about 130 miles east of San Francisco.