The HH-1N Huey crashed about 20 miles north of Yuma near the Colorado River on Thursday. The wreckage was discovered early Friday, and three Marines and one Navy sailor were pronounced dead at the scene, said 1st Lt. Rob Dolan, a spokesman for the Marine Corps Air Station in Yuma.
An injured Marine was transported to Yuma Regional Medical Center by another search-and-rescue helicopter from the base and is listed in stable condition.
The names of the dead and injured will not be released for 24 hours under standard military policy, Dolan said.
The chopper assigned to the Marine Corps Air Station in Yuma was last heard from at about 4 p.m. Thursday, Marine Sgt. Ryan O'Hare said.
The aircraft was flying on a routine training mission near the Army's Yuma Proving Ground, a sprawling 1,300-square-mile military reservation along the Arizona-California border used to test combat systems and as a helicopter test center.
The missing aircraft was assigned to the Headquarters and Headquarters Squadron at the Marine Corps Air Station. The Yuma facility is the home base for several air squadrons and is the world's busiest Marine Corps air station.
In 2005, a Marine Corps Harrier jet crashed into a neighborhood near the air base but the pilot had only minor injuries and no one on the ground was hurt.
Hueys, first produced in 1956, are Vietnam War-era workhorses. New aircraft are scheduled to replace them beginning in 2008.