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The thrust reverser theory has been all but thrown out, and it appears an engine on USAir Flight 427 didn't break loose, either.

New evidence in the crash has put investigators searching for a cause back at square one - and they aren't ruling out anything."You go up blind alleys, but you have to go up them to see that they they were blind alleys," Tom Haueter, chief investigator for the National Transportation Safety Board, said Tuesday. "We are still looking at every possible avenue."

The Boeing 737-300 dropped from 6,000 feet Thursday and nose-dived into a wooded ravine at 300 mph, killing all 132 people on board. Investigators believe the plane's right wing may have risen, forcing the plane to roll to the left and go out of control.

Federal investigators initially speculated that the thrust of the jet's right engine was suddenly reversed. Some of the engine's actuators, devices that engage the thrust reversers, were found in the deployed position.

A closer inspection showed the actuators probably shifted on impact, suggesting that the engine was operating normally when the crash happened.

Jets use reverse thrust to slow down after landing.

Also, a close inspection of the right engine's rear mount, which was located Tuesday, cast doubt on the theory that the engine came loose from its position beneath the wing, Haueter said. Damage on all engine mounts was consistent with crash damage.

Despite the new evidence, investigators are continuing to test those scenarios and others on a computer model of the plane. Metallurgists will be asked to examine the wreckage.

One theory still under consideration is that the spoilers - wing flaps used to slow a plane in flight or after landing - worked unevenly. No evidence has been found to indicate the spoilers were malfunctioning.

No other theories have emerged to explain the crash, although investigators are studying whether a possible autopilot malfunction might have contributed to it, Haueter said.

Haueter said he could not confirm a New York Times report that the plane's cockpit recorder taped a strange, bumping noise, followed by a voice asking, "Jeez, what was that?"

Haueter said about 90 percent of the wreckage has been recovered and taken to a hangar where the plane is being reassembled or sent to laboratories.

He said pilots of planes flying near the jet said they did not see the crash and saw no flocks of birds, which can damage engines if sucked inside. The plane appeared to be carrying no hazardous materials.

Airline workers who spoke to crew members before the plane left Chicago said they appeared alert and in good spirits.