ENSENADA, Mexico (AP) _ A small airplane crashed Saturday at a military base in Baja California state, killing all six Americans on board, police said.

The plane went down about 11 a.m. while trying to land at the Area 3 military air base outside Ensenada, a coastal city about 50 miles south of the border with California, said police official Jesus Luna. No one on the ground was injured or killed, he said.

Another police official later confirmed that all six people on board were from the United States. He could not be cited by name because of police regulations. He said he had no further details.

The state news agency Notimex reported that the six were all American members of a medical assistance group called Sky Doctors.

Police commander Jesus Eduardo Velez said the plane was leaking fuel and was on fire at the crash site, in a remote mountainous area less than a half mile from the airport runway, Notimex reported.

Air base commander Sergio Alejandro Escobar told Notimex the plane was en route from San Ignacio, about 470 miles south of Ensenada, and had received authorization to land from the control tower.

Escobar said the crew never reported an emergency, but that initial indications were that the plane had engine problems prior to the crash.

A witness to the accident, Romel Ramirez, who was working at a factory nearby, told Notimex that he saw the airplane do a somersault in the air and then heard the engine cut out.

In May, three Northern California volunteers for a medical assistance group died when their single-engine plane plunged into the Pacific Ocean on a return trip from a weekend mission in Baja California.