A United States Marine Corps aircraft crashed while landing at an airport west of Tucson, Arizona, and was engulfed in flames, killing all 19 people on board, authorities said.
The plane went down Saturday night with four crew members and 15 passengers aboard, said Gunnery Sgt. Nathan Portman of the Marine Corps Air Station at Yuma, where the flight originated, about 390 kilometers (240 miles) away.
All the victims were Marines, but officials said they did not know where they had been based. The aircraft was not based at the Yuma air station.
The MV-22 tiltrotor Osprey, which looks like a large turboprop plane but can take off like a helicopter, was landing at the Marana Northwest Regional Airport as part of a training mission when it crashed, said Marine Col. William D. Catto.
The plane was expected to return to the Yuma air station, Catto said.
Another Marine spokesman, Corp. Jason Schaap, said the Osprey that crashed and a second one were simulating the evacuation of civilians. Schaap didn't immediately have other details on the exercise.
Firefighters said witnesses reported seeing the plane head "straight downward," said Katy Heiden, spokeswoman for the Northwest Fire District.
"It was fully engulfed and there were small explosions," she said.
Catto said there were no weapons on board the plane.
"This terrible loss of life is a reminder of how many men and women in the nation's military put their lives at risk, each and every day, so that we might be a free people, and the cause of peace can be advanced throughout the world," U.S. President Bill Clinton said in a statement issued by the White House.
Introduced last September, the Marine Corps' Osprey is also known as a convertiplane. Its two propjet turbines power two oversize propellers.
The Osprey flies at twice the speed, has twice the range and carries twice the payload of the Vietnam-era CH-46 helicopters it will replace in the Marine Corps' inventory.
Jointly produced by Bell Helicopter Textron of Fort Worth, Texas, and Boeing Co., in Ridley Park, Pennsylvania, the aircraft can achieve speeds over 645 kph (400 mph) and an altitude of 7,500 meters (25,000 feet). It is designed to carry up to 24 troops.
Boeing spokeswoman Susan Bradley said late Saturday night that it was not yet clear whether the company would join the investigation.
Military planners see the aircraft as a means of getting more U.S. troops and pilots safely out of danger zones and enhancing drug interdiction, humanitarian and civilian rescue capabilities.
Critics have assailed the aircraft's high cost and safety. Early safety concerns plagued the innovative aircraft, but builders say modifications from the original design have made it lighter and safer.