The Harvard Yacht Club Building, site of a helicopter crash which killed all four passengers Wednesday morning, remained roped off by yellow police tape yesterday.

But a federal investigation into the cause of the tragic decent, which left two state troopers and two AT&T employees dead, has already revealed that the crash may have been caused by transmission failure in the six-seat American-made helicopter.

According to eyewitness accounts of yesterday's disaster, only the main rotor of the helicopter was not operating at the time of the crash.

"The only thing running was its tail section," said MIT Utilities Construction Coordinator David M. Barber, who witnessed the crash from across the street.

Larry R. Retta, a spokesperson for the Army Safety Center at Ft. Rucker, Ala, said yesterday that the failure of the main rotor may have been due a faulty transmission.

"When the motor is not turning, it's a sign of transmission failure," Ketta said. "That's what it sounds like it was to me."

On Wednesday, the helicopter's wreckage was transported on a flatbed truck from the crash site to a hanger at Logan International Airport, where federal investigators are working to determine the cause of the deadly crash.

The team of investigators have recovered several key pieces of the aircraft, including its control panel, engine, and tail section, Alan Yarman, lead investigator for the National Transportation Safety Board (NTSB), said at a press conference yesterday.

"We are setting the aircraft up, reconstructing it to the way it was prior to the accident," Yarman said. "There's still a lot of areas we want to look at."

'Like a Pinwheel'

Nearly all helicopter crashes are the result of engine malfunction which cut off power to the rotor blades, said Robert E. Breiling, an independent contractor who has compiled aircraft safety records since 1963.

When an engine malfunctions, the pilot can keep the craft from plummeting to the ground by forcing air through the engines in a technique known as autorotation, Breiling said.

"It's like with a pinwheel," Breiling said. "When the engine fails, you shift it into neutral, then let the air keep run through the blades and keep it spinning."

Most emergency autorotation landings are not fatal, and pilots must master the procedure during flight school, he said.

John D. Sturgeon, who saw the accident as he stood outside his Memorial Drive office building Wednesday, said the pilot appeared have lost control of his craft.