The U.S. National Transportation Safety Board is expected to approve a final report into the crash of Trans World Airlines Flight 800, which exploded shortly after taking off from New York four years ago, although the report doesn't pinpoint the exact cause of the crash.

The 700-page report stops short of explaining what ignited the explosion in the Boeing 747's center fuel tank 11 minutes out of Kennedy International Airport, killing all 230 passengers and crew members.

Researchers tried to pinpoint where in the tank the blast originated, yet ``there were just too many variables,'' one investigator said. There are just too many things that do not go together and still one question that can't be answered, why this has only happened once.