The servicemen were attempting to land an MV-22 Osprey when it went down. The aircraft has been dogged by safety concerns.
A Marine Corps aircraft in the final stages of its introduction crashed in Arizona, killing all 19 Marines on board and renewing questions about its safety.
The MV-22 Osprey plunged from the sky and exploded at a small regional airport in Marana, Ariz., northwest of Tucson, as it was preparing to land Saturday night, said Marine spokesman Capt. Rob Winchester. The Osprey is a technology hybrid with rotors that swivel so that it can take off like a helicopter but cruise like a turboprop commuter plane.
It is a new generation of aircraft scheduled to eventually replace all of the Marines' primary troop-transport helicopters. The military began flying the aircraft six months ago, and the Marine Corps temporarily halted their use Sunday after the crash.
This was the third crash of an Osprey in the past decade. No one was killed in the first, in Delaware in 1991, but in the second, in Virginia in 1992, all seven people on board were killed after an engine caught fire.
On Sunday, investigators were reviewing the crash site. Few details were released, but officials said the mission was conducted with night vision goggles and infrared radar. Goggles allow crew members to see in the dark but can sometimes impair peripheral vision.
The aircraft had been attempting to land at the airport when it crashed. It was one of two Ospreys simulating the evacuation of civilians, similar to what Marines would do if they were removing people from an embassy in a hostile country.
Carol Ward, who lives about 5 miles from the airport, said she watched the plane fly by from her porch. It disappeared behind a mountain and a few seconds later "I saw the smoke and this big old poof," she said.
The dust from the crash "just eliminated the sky," she said.
Firefighters said witnesses reported seeing the plane head straight down and become engulfed in flames after it crashed.
Some witnesses said they thought the plane was on fire before the crash.
The four crew members in Saturday night's crash were from a task force headquartered in Quantico, Va.
The passengers were 14 Marines from 3rd Battalion, 5th Marines based at Camp Pendleton, Calif. _ including Cpl. Can Soler, 21, a rifleman from Palm City, Fla. _ and one from Marine Corps Air Station-Miramar in San Diego County, according to the Marine Corps.
Controversy has dogged the Osprey program since its inception in 1981. The Marine Corps has championed the Osprey as a faster, larger and more flexible replacement for its Vietnam-era CH-46 Sea Knight troop and cargo transport helicopters.
Marine commanders envision the Osprey as the ideal long-range aircraft over the next 25 to 50 years for ferrying Marines from Navy ships into combat zones, humanitarian operations or rescue missions.
Built jointly by Bell Helicopter Textron of Fort Worth and the Boeing Company in Ridley Park, Pa., the Osprey can reach speeds of more than 400 mph and an altitude of 25,000 feet. It is designed to carry up to 24 people or external loads of 15,000 pounds.
But at $44-million, the Osprey has drawn sharp criticism from many military analysts for its steep cost as well as its safety record.
A report by the General Accounting Office, an investigative arm of Congress, determined in 1990 that the Osprey was too heavy and vibrated excessively in early tests, but builders say modifications from the original design make today's Ospreys lighter and safer.
President George Bush tried to kill the program in 1991. But Congress, influenced by the Marines' powerful lobbying effort, kept the program alive.
Clinton eventually authorized the purchase of up to 458 planes for the Marines, Air Force and Navy, with 360 of them going to the Marines.
The Marines had only five Ospreys in use: four out of Yuma, including the one that crashed, and one based at the Marine Corps Air Station, New River, N.C.
Pentagon spokeswoman Capt. Aisha Bakkar-Poe said the Marine Corps' other four Ospreys will not be flown until "we can get our arms around what may have happened."
_ Information from the Associated Press and the New York Times was used in this report.