Investigators strongly suspect that the Jan. 21 crash in Reno that killed 70 persons on a Galaxy Airlines plane occurred because a service door was left open on the airplane's wing and prompted severe vibrations, causing the pilot to cut engine power below safe limits, sources said yesterday.

The recording of the flight crew's conversation moments before the crash was released yesterday by the National Transportation Safety Board. It clearly indicates that the Lockheed Electra stalled -- stopped flying -- almost immediately after takeoff.

"We need more power," Capt. Allen D. Heasley said 29 seconds before the plane hit the ground.

Then the computerized device installed in all commercial airliners to warn pilots they are too close to the ground began its whooping alarm, commanding: "Pull up! Pull up!"

"A hundred knots," copilot Kevin Field said, reading the air speed from his instruments. A speed of 100 knots is insufficient to keep an Electra airborne.

"God, God," flight engineer Mark Freels said.

Heasley ordered "max power." But it was too late.

Half a minute before that conversation, Heasley had ordered the throttles on the four-engine turboprop pulled back, presumably in reaction to a noise, described in the safety board transcript as a "thunk."

Then he told copilot Field to radio the Reno tower and "tell them we have a heavy vibration."

Investigators have been seeking the source of the vibration almost from the moment of the copilot's transmission. They now know that all engines were functioning properly and that there were no difficulties with fuel feed systems or propellers, which had been suspects.

Other Electra pilots have been interviewed and have reported severe vibration when a service door, the "ground start access door," on the front underside of the left wing is left open. The door is hinged on the side closest to the front of the wing and fixed at the rear with two fasteners.

It is opened when the plane is on the ground. A hose is attached to a fitting inside the door so the air system from one engine can be pressurized to ease starting. Investigators have been interviewing ground personnel at Reno to determine if anyone can remember whether the door was closed and fastened. Sources said the interviews have been inconclusive.

The cockpit voice recording shows that the "thunk" began while the plane was rolling toward takeoff. First, according to the transcript, there was a "sound similar to glare shield rattle," then, just as the plane reached go/no-go speed on the runway, the first "sound of thunk."

As the landing gear was retracted, Heasley asked flight engineer Freels, "What is it, Mark?"

"I don't know. I don't know, Al," Freels replied.

Freels then reported that the engines had reached "METO," maximum speed except on takeoff.

"Okay," Heasley said, "Pull them back from METO." He ordered reduced power at one of the most critical points in flight, then immediately said, "Tell them we need to . . . get it back on the ground."

That decision came about one minute after the 1:04 a.m. takeoff. The charter flight was carrying Minneapolis area residents home after a Super Bowl and gambling weekend. Two of three survivors died in the two weeks after the crash. The third, a youth whose father was killed in the crash, is reported doing well.

Ten seconds after Heasley said, "Tell them we have a vibration," the transcript reports a "sound similar to stall buffet onset, [and] loose cockpit equipment begins to rattle."

Stall buffet is a natural aerodynamic phenomenon that occurs when a plane starts to move too slowly through the air to sustain flight. On most aircraft, the buffet is quite noticeable. The typical pilot's reaction is to increase power and lower the nose.

However, when the stall occurs on takeoff, lowering the nose is not an option. An increase in power, the only alternative, apparently came too late for the Galaxy flight.

The plane crashed in an open field southwest of the airport and immediately burst into flames. It was the first major commercial airliner crash in the United States in more than two years.