(AP) A fire likely 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 a problem with the wiring to power the system went undetected and the spark "most likely" started there.

It said the U.S. Federal Aviation Administration's certifying system failed to ensure that the entertainment system was designed and installed properly for the MD-11. However, chief investigator Vic Gerden said the wiring flaw in the entertainment system was not the cause of the spark that started the fire.

The report included nine safety recommendations involving testing for insulation materials and electrical systems, and improving the flight cockpit and data recording systems.

Safety board investigators previously made 14 recommendations that led to the removal of flammable insulation material from aircraft and improved fire reaction measures for pilots.

"This has been the largest, most complex aviation investigation the TSB has ever undertaken," safety board chairman Camille Theriault said. "The efforts of thousands of hardworking people from various countries, industries and regulatory authorities have culminated in a comprehensive report that has changed the face of aviation safety."

Swissair Flight 111 plunged into the Atlantic Ocean off the Nova Scotia coast 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.

Since the crash, Swissair has gone out of business in the wake of the Sept. 11, 2001, terrorist attacks in the United States that crippled the airline industry.

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 the recommendations include measures to certify additional systems such as the in-flight entertainment system and raise industry standards for resetting circuit breakers.

Gerry Einarsson, a former Canadian government transport engineer, recently said the entertainment system — which allows passengers to view videos and play computer games — required excessive amounts of operating power that resulted in higher cabin temperatures.

The report noted that a check of 15 other Swissair MD-11s showed the wiring for the entertainment system was installed differently on four of them.

"The overall result," it said, was that the FAA's certification of the entertainment system "did not ensure that all the required elements were in place to design, install and certify a system that would be compatible with the MD-11 type certificate."

A lack of smoke or fire detection and suppression devices, which were not required at the time, left the crew with few resources, the report said. The pilots had no chance to try an emergency landing.

"We have concluded that, even if the pilots could have foreseen the eventual deterioration due to the fire, because of the rapid progression of the fire, they would not have been able to complete a safe landing," Gerden said.

Investigators used 2 million pieces of recovered wreckage, some as small as a dollar coin, to partially reconstruct the jetliner.

Three years ago, the Canadian safety board recommended that airlines do a better job of training and equipping crews to detect and fight fires on planes.

An earlier recommendation by investigators noted safety problems with the plane's insulation blankets, which have been suspected of spreading fire. The FAA in the United States responded by requiring that insulation blankets be removed from all U.S.-registered MD-11 aircraft.

In March 2002, a U.S. federal judge dismissed claims for punitive damages for families of victims of the Swissair crash.