HUNTSVILLE, Texas—Four people were killed when a medical helicopter transporting a patient crashed Sunday in the Sam Houston National Forest, authorities said.

The cause of the crash was still under investigation, and officials said rescued crews struggled to find the wreckage in the densely wooded rural area.

The PHI Air Medical helicopter was headed from a Huntsville hospital to another in Houston when a missed check-in with dispatch raised concern, company spokesman Jonathan Collier said. The aircraft was found about 10 miles from where it had taken off, he said.

“This is a devastating loss,” Collier said.

Killed were flight nurse Jana Bishop, pilot Wayne Kirby and flight paramedic Stephanie Waters, all PHI employees.

The name of the patient on board was not immediately released.

Ronald Herwig, a spokesman for the Federal Aviation Administration, said the crash happened about 2:45 a.m. He said federal safety investigators were headed to the crash site, located about 70 miles north of Houston.

Collier said the crew on board was “experienced” but did not immediately have more details about their background.