The fire had grown to 14 hectares by midday on Thursday amid dry, windy conditions. County officials were concerned about the blaze and local agencies were working with the US Forest Service to contain it.
The cause of the crash was unknown.
The plane was located on Thursday morning in steep, rocky terrain in the Capitan Mountains that was difficult to access, with crews hiking the last half-mile to reach the crash site, he said.
The victims were flight crew and medical personnel. Their names have not yet been made public.
The flight departed from Roswell Air Center and was headed to Sierra Blanca Regional Airport, the Federal Aviation Administration said in a statement.
The FAA and National Transportation Safety Board will investigate the crash.
The plane operated by Trans Aero MedEvac had been on a medical transportation mission and was reported overdue after communications and radar contact were lost, the company said in a statement.
Trans Aero MedEvac has operated in southeastern New Mexico and west Texas since 1966.
Ruidoso, a mountain town with a year-round population of less than 8000, sits at the base of south-central New Mexico's Sierra Blanca range. The surrounding area, which includes Lincoln National Forest, is heavily forested and rural.
Prior to Thursday's crash, there were 25 fatal crashes of medical planes over the past 25 years that killed nearly 70 people, according to NTSB records.
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