STRATFORD, Conn. (AP) _ As dozens of people watched in horror, a $25 million military helicopter that was to provide cargo service for the White House lost its blades, flipped backward and slammed into the ground, sending flames hundreds of feet into the air.

The four people on board, all Sikorsky Aircraft employees, were killed in the crash Thursday at the company's airfield. The transport helicopter was undergoing a flight check before it was to be added to the White House fleet.

The company today identified the victims as pilots David A. Kish, 39, and Yong H. Lee, 39; crew chief Leonard R. Kuderna, 47; and Thomas L. Payne, 52, an electronics flight systems technician.

No one on the ground was hurt.

The cause of the crash was not immediately known. It was not clear if the helicopter had just taken off or if it was attempting to land.

Dozens of people working nearby and passing motorists saw the crash.

``It lost its blades. It did a back flip, and then there was a thick, black cloud of smoke. It went down so fast,'' said John Nasufi, owner of Stella's Restaurant across the street from the plant.

The heavy-lift helicopter was to be assigned to a Marine unit at Quantico, Va., that provides support services for the White House, Sikorsky spokeswoman Gina McAllister said. It would have been used for such purposes as ferrying limousines and other advance equipment for the White House.

The helicopter, however, would not have been used to carry the president.

Lt. Scott Gordon, a Marine Corps spokesman, did not know when the helicopter was due for delivery. He said it was in the beginning stages of its flight acceptance test, which typically involves radar and communications equipment, among other areas of the craft.

The CH-53E, built by Sikorsky and known as the ``Super Stallion,'' is a three-engine helicopter that can carry 56 people and is used by the Marines and Navy.

The last major problem with the helicopter _ defined by the military as one that caused a death or more than $1 million in damage to the aircraft _ was a rough landing in June 1993. There were no fatalities.

Its accident record in the mid- and late '80s was much different. Seven crashes killed 19 people between 1984 and the spring of 1987, leading the Navy to ground the aircraft twice that year for inspections of engine gearboxes. They were grounded briefly again in 1988 because of a defective nut.

Sikorsky paid $9 million for a two-year review, then reported in 1989 that the Super Stallion was safe.