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HALIFAX, Nova Scotia (CNN) -- Evidence collected by investigators suggests that a catastrophic electrical failure aboard Swissair Flight 111 may have led to the crash last week.

Canadian investigators said Tuesday that preliminary investigation of fragments recovered from the cockpit revealed signs of heat stress that could explain why smoke billowed around the pilots before the jetliner crashed.

"There are some signs of heating on some of the small pieces of the wreckage retrieved," said Vic Gerden, chief investigator for Canada's Transportation Safety Board. "The limited amount of wreckage retrieved to date indicates this area of heat stress is in the cockpit.

"But it is much too early in the process to draw conclusions from this," he said.

Flight 111 crashed into the Atlantic off Nova Scotia on September 2, more than an hour after leaving New York for Geneva, killing all 229 people aboard.

Gerden said he hopes there will be valuable data on the plane's recovered flight data recorder, even though it stopped working during the final six minutes before the plane crashed.

"The most crucial area is not the last six minutes, as some may think, but prior to those six minutes," he said.

He said the recorder might show how systems aboard the plane failed in the 10 minutes between the pilots' initial distress call and the time the machine shut off.

The lengthy silence between the plane's last communication with air traffic controllers and its estimated impact with the water has led investigators to speculate there could have been a serious system failure aboard the airliner.

Gerden said Monday that "There is a strong possibility that there weren't electrics on the airplane."

He hinted that a total electrical failure could have left the two pilots to fly a crippled plane manually with little more than a mechanical compass at their disposal.

Thick smoke and fumes from an electrical fire also could have hampered the pilots' efforts to operate the plane's rudders and flaps manually, and to land the fuel-heavy jetliner at Halifax airport.

The possibility of electrical failure has led investigators to focus on reports of problems with the insulation tape on the wiring in some MD-11s.

The Boeing Co. has confirmed that the tape was used on some of the planes, which were built by McDonnell Douglas. But Boeing, which merged with McDonnell Douglas last year, said it stopped using the insulation for economic reasons several years ago.

The recovered flight data recorder (top) and the interior portion that contains the data

The U.S. Navy banned the tape from its jets in the early 1990s after the material showed a tendency to crack and spark.

"Obviously, if it was used, that's a possibility that we must consider, and we'll be looking closely at electrical bundles as they are brought up," said John Maxwell, an official with Canada's Transportation Safety Board.

Gerden also released a more complete version of the last conversation between the Swissair pilots and an air traffic control tower in Moncton, New Brunswick.

Expanding on excerpts released earlier, the transcript showed that the pilots reported putting their oxygen masks on and telling the controllers they would have to fly the plane manually.

Gerden repeated that he saw nothing in the conversation to suggest unwise decisions or unprofessional conduct.

"The crew conversation was done in a professional manner throughout," he said, stressing that it would be premature to pass judgment until after all evidence was examined.

Some lawyers and relatives of victims have suggested that the pilot, Urs Zimmermann, should have made a direct, though possibly risky, descent to the Halifax airport after reporting smoke. Instead, he descended gradually, making several turns to lose altitude and dump fuel.

In Zurich, Switzerland, Swissair's chief pilot-designate, Rainer Hiltebrand, rejected those suggestions as irresponsible.

Investigators hope to get further information from the plane's cockpit voice recorder, which is still on the ocean floor. A signal from that recorder has been detected, but bad weather Tuesday forced a halt in diving operations.

The divers were expected to return to work Wednesday, supported by the arrival from the United States of the Navy salvage and rescue ship USS Grapple.

The Grapple helped with the undersea recovery of wreckage of TWA Flight 800 off Long Island in 1996. It has a team of more than 30 divers on board, as well as equipment for lifting large wreckage from the seabed.

Also on hand is a 16-member U.S. Navy team from Panama City, Florida, which has brought advanced sonar and imaging equipment to provide greater detail of the ocean floor.

The Associated Press and Reuters contributed to this report.