(CNN)Investigators may have hit a dead end in their quest to say with certainty whether a piece of airplane debris found in the Indian Ocean comes from a Malaysian passenger jet that disappeared mysteriously in 2014.

The Malaysian Prime Minister said a few weeks ago that the debris clearly is from MH370, which disappeared with 239 people aboard in March 2014. Investigators in France said there were "very strong presumptions" that the debris, called a flaperon, was from MH370, but that further testing was needed to say that with ironclad confidence,.

Now comes word that a Spanish company has told French investigators that it cannot tell with certainty from consulting its records whether the flaperon found on Reunion Island came from MH370, a French source close to the investigation told CNN. The flaperon is from a Boeing 777 -- and MH370 was a Boeing 777, the only one in the world that's unaccounted for -- but the company can't say with absolute certainty that the flaperon found on Reunion Island comes from the missing plane, the source said.

The Spanish company manufactured part of the flaperon. Investigators had hoped to match a number found on the debris from Reunion Island with records from the Spanish company -- to confirm that the debris did, in fact, come from MH370 -- but that proved impossible.

The development on Monday does not mean that the debris found on Reunion isn't from the missing plane -- it just means that investigators cannot yet make the link with 100% certainty.

But even the slightest possibility that the debris might not be from the missing plane adds to what so far ranks among aviation's greatest mysteries.

In the early hours of March 8, 2014, Flight 370 took off from Kuala Lumpur International Airport in Malaysia en route to Beijing, with 239 passengers and crew on board.

At 1:19 a.m., as the Boeing 777-200ER was flying over the South China Sea, Malaysian air traffic controllers radioed the crew to contact controllers in Ho Chi Minh City for the onward flight through Vietnamese airspace.

The crew's acknowledgment of the request was the last thing ever heard from MH370: "Good night Malaysian three-seven-zero."

Shortly afterward, air traffic controllers in Malaysia lost contact with the plane somewhere over the sea between Malaysia and Vietnam.

The aircraft's transponder, which identifies the plane and relays details like altitude and speed to controllers, stopped transmitting. MH370 seemingly disappeared without a trace.

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Malaysian authorities revealed later that military radar had tracked the plane as it turned back to the west and flew across the Malaysian Peninsula, up the Strait of Malacca, before flying out of radar range at 2.14 a.m. and vanishing once again.

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http://www.cnn.com/2015/08/31/world/mh370-investigation/index.html