HALIFAX, Nova Scotia A fire probably caused by an electrical spark crept undetected along the insulation of Swissair Flight 111 four years ago, giving the pilots no chance to save the 229 people on board, investigators concluded Thursday.

Canada's Transportation Safety Board ended its largest investigation ever, lasting more than four years and costing more than $30 million, without being able to declare unequivocally what caused the fire that brought down the MD-11 airliner off the coast of Nova Scotia on Sept. 2, 1998.

The 338-page report focused on the in-flight entertainment system aboard the McDonnell-Douglas aircraft, saying that a problem with the wiring to power the system had gone undetected and that the spark "most likely" started there.

It said that the U.S. Federal Aviation Administration's certifying system had failed to ensure that the entertainment system was designed and installed properly for the MD-11. But Vic Gerden, the chief investigator, said that a wiring flaw detected by investigators in the entertainment system did not spark the fire.

Swissair Flight 111 plunged into the Atlantic Ocean off Nova Scotia while en route from New York to Geneva. Pilots reported smoke in the cockpit 53 minutes into the trip, and the electrical systems began failing 13 minutes later.

The report provided a clinical and sobering account of the tragedy, telling how a spark in the wiring of the in-flight entertainment system probably started a fire that slowly burned its way through insulation above the cockpit.

"It was determined that the fire most likely started from an electrical arcing event that occurred above the ceiling on the right side of the cockpit," the executive summary said.

It offered no specific reason for what caused the spark, but recommended measures to certify in-flight entertainment system and raise industry standards for resetting circuit breakers.