HALIFAX, Nova Scotia - An electrical spark in the wiring of an in-flight entertainment system likely started the fire that brought down Swissair Flight 111 four years ago, and pilots had no chance to save the 229 people on board, a long-awaited report concluded Thursday.

But the investigation, the largest in Canadian history, ended without determining unequivocally what caused the blaze that fed undetected on insulation above the cockpit.

Flight 111 plunged into the Atlantic Ocean off the Nova Scotia coast Sept. 2, 1998, 74 minutes into its New York-to-Geneva trip. Pilots reported smoke in the cockpit 53 minutes into the flight, and the electrical systems began failing less than 15 minutes later.

The comprehensive safety board report contained imprecise language when referring to the cause of the fire. The spark between a wire in the plane's in-flight information system and another wire is referred to as arcing.

"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 summary said.

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," chief investigator Vic Gerden said.

The four-year, $40 million investigation reconstructed almost the entire MD-11 aircraft from 2 million pieces of debris, some as small as a dollar coin, in what the brother of one victim called a "Herculean task."

Flight 111 took off from New York's John F. Kennedy International Airport at 8:17 p.m. EDT. The crew detected an unusual odor in the cockpit 53 minutes later, and smoke was visible in the cockpit three minutes after that.

The plane tried to divert to Halifax, Nova Scotia, and began dumping fuel. Its electrical systems then failed and, at 9:31 p.m., it hit the water off Peggy's Cove, Nova Scotia, at almost 350 mph, killing everyone on board.

Investigators' 338-page report focused on the in-flight entertainment system aboard the McDonnell-Douglas aircraft, saying a flaw in its installation went undetected and the spark "most likely" started in the system's wiring.

In addition, the report cited shortfalls in the U.S. Federal Aviation Administration's certification of the entertainment system, which allowed first-class and business-class passengers to watch videos and play video games. It said the FAA failed to ensure that the system for the MD-11 was designed and installed properly.

Gerden, however, said there was no apparent connection between the installation flaw and the spark.