Information gathered by investigators probing last month's explosion of a Thai Airways 737 strongly suggests that the plane's center fuel tank exploded without being ignited by a bomb or other external device, but the Government of Thailand's probe hasn't ruled out sabotage yet.

"Physical evidence has been found that the center wing tank exploded," said a U.S. National Transportation Safety Board (NTSB) statement. "Although chemical traces of high energy explosives were initially believed to be present, samples have been submitted to the FBI for confirmation by laboratory equipment that is more sensitive than equipment available in Thailand.

"Although a final report has not yet been issued, the [U.S. Federal Bureau of Investigation] has found no evidence of high explosives in any of the samples tested to date," NTSB continued. "Despite a thorough examination of the wreckage by Thai and American bomb experts, no physical evidence of a bomb has been found to date."

NTSB is one of several organizations assisting the Thai government in the investigation.

The March 3 explosion of Flight 114 killed one flight attendant and destroyed the plane, a nine-year-old 737-400, as it was being prepared for a flight from Bangkok's Don Muang International to Chiang Mai. Initially, officials believed the blast was caused by a bomb intended to kill Thai Prime Minister Thaksin Shinawatra, who was a passenger on the flight.

But the lack of explosive traces and several other factors have investigators focusing on a scenario similar to the one that investigators believe played out in TWA Flight 800, in which a Boeing 747 was destroyed off the coast of Long Island in July 1996.

The longest and most expensive NTSB aviation accident probe in history concluded that the TWA plane exploded when fuel vapors in the plane's center tank were ignited - most likely by a short-circuit that jumped from high-voltage wires external to the tank to low-voltage wires that ran into the tank, the board determined. The vapors were likely heated to a flammable level due to a combination of heat generated by air conditioning packs located directly under the tank and the hot weather on the ground the day the 747 departed, NTSB said.

The Thai aircraft's center tank exploded at 1448 local time in weather that was about 90 degrees Fahrenheit. Air conditioning packs located directly beneath the center wing tank "had been running continuously since the airplane's previous flight, including about 40 minutes on the ground," investigators said. About 18 minutes after the initial blast, the plane's right wing tank exploded.

NTSB has compared sound signatures from Flight 114 to ones pulled from a Philippine Airlines 737-300 that had an explosion start in an empty center wing tank in May 1990. "Neither recording includes a precipitating sound of an initiating explosion that may have ignited the fuel tank," NTSB said.

Safety officials never determined with certainty what ignited the Philippine plane's vapors. While investigators caution that nothing has been ruled out in the Flight 114 probe, the incident could increase pressure on regulators to address certain certification and operational issues regarding fuel tank and wiring safety.

The TWA probe underscored threats that damaged wiring could pose in an aircraft, such as acting as a potential ignition source. While the TWA plane was considerably older than the Thai jet, investigators found that wire chafing and damage caused by foreign objects - such as drill shavings - were often found in new-build models.

Of greater concern to NTSB has been finding ways to eliminate the explosive mixtures in fuel tanks. Evidence uncovered during the TWA probe prompted NTSB to put the issue on its "most wanted" improvements list in December 1996. The board made recommendations calling for operational changes to help limit the potential of explosive fuel-air mixtures in tanks and reduce temperatures inside fuel tanks.

FAA has since issued more than 40 aircraft-specific rules aimed at eliminating ignition sources in fuel tanks and proposed sweeping changes to certification and continued airworthiness standards, but told NTSB in a November 1999 letter that it has found "no practical means of reducing fuel temperatures in fuel tanks to the extent where benefits could be achieved." FAA told the board it would continue to study the matter.

The explosive-mixture issue remains on NTSB's most-wanted list. The board also has asked FAA to review certification standards that permit heat-generating equipment like air conditioning packs to be located near fuel tanks.