STRATFORD, Conn. (AP) _ A military transport helicopter undergoing a flight check before it was to be added to the White House fleet crashed and burned Thursday at Sikorsky Aircraft, killing all four crew members.

Dozens of people working nearby and passing motorists watched in horror as the huge Sikorsky-built Super Stallion labored above the airfield, then dropped straight to the ground.

``I saw it start going up, and the next thing there was a big boom _ it shook our building, it crashed, and there was smoke billowing out all over,'' said Mary Beth Desrosiers, a clerk at a pharmacy across the street.

The dead were all Sikorsky employees. Their identities were not immediately released.

The cause of the crash was not immediately known, said Sikorsky spokeswoman Gina McAllister.

The CH-53E helicopter was to be assigned to a Marine unit at Quantico, Va., that provides support services for the White House, McAllister said. The helicopter would not have been used to carry the president, she said.

The Connecticut Post said the aircraft would have been used for such purposes as ferrying limousines and other advance equipment.

The Super Stallion is a three-engine, heavy-lift helicopter that can carry 56 people and is used by both the Marines and Navy.

Witnesses said the helicopter crashed just after takeoff. They gave conflicting accounts as to whether it exploded in the air or when it hit the ground.

Jim Dennison, a surveyor working nearby, said he watched the crash unfold through his viewer.

``The propeller came off first, and it looked like it hit the building. Then it went sideways. Then I noticed pieces flying. Then it dropped out of my field of view,'' he said.