KUALA LUMPUR — (CNN) When a piece of plane debris was found last week on a remote French island in the Indian Ocean, family members of Chinese passengers on missing Malaysia Airlines Flight 370 made it clear what they expected: Certainty.

In a nearly 17-month-long search that has been plagued with false alarms, the relatives hoped to avoid yet more agonizing confusion. Was the wing component the first piece of physical evidence to be recovered from the missing Malaysian aircraft or not?

“We do not want to hear guarantees of 99% likelihood from certain authorities,” the Chinese families said in a statement. “We need confirmation of 100% certainty.”

But between the announcements from Malaysian and French authorities about the origin of the object, which was discovered on Reunion Island, the relatives have once again been left with some room for uncertainty.

Malaysian Prime Minister Najib Razak said the wing part, known as a flaperon, has been “conclusively confirmed” to be from Flight 370.

Paris Deputy Prosecutor Serge Mackowiak, however, used slightly less definite language, saying that there were “very strong presumptions” that the debris was from the Malaysian plane but that further tests would need to be carried out to confirm it.

Najib made his announcement in the early hours of Thursday morning in Malaysia, saying he hoped that “this confirmation, however tragic and painful, will at least bring certainty to the families and loved ones of the 239 people on board MH370.”

He didn’t go into detail about what it is that Malaysian officials, who are in charge of the investigation into the plane’s disappearance, believe provides the conclusive link.

Mackowiak spoke soon afterward in Paris, explaining the basis for the strong suppositions that the flaperon is from Flight 370.

Boeing officials have concluded that the part’s technical features — such as its color and the structure of its joints — confirm that it’s from a 777 aircraft, the same model as MH370, he said.

Drawing on documentation provided by Malaysia Airlines, the prosecutor said, experts also identified “common technical characteristics” between the flaperon found on Reunion and that of Flight 370. But he stopped short of making a definitive link.

The softer language used by the French prosecutor highlights the complex, multinational nature of the investigation into what happened to MH370, which disappeared on March 8, 2014, with 239 people on board.

Malaysia Airlines said in a statement that the confirmation announced by Najib, which it described as “a major breakthrough,” had been made jointly by French, Malaysian, Chinese and Australian officials.

But a source close to the investigation told CNN that French and U.S. experts examining the flaperon have not yet found anything that would definitively link it to Flight 370. The American officials involved are from the National Transportation Safety Board.

While it is expected that the wing part will eventually be matched to MH370, the source said that there was so far nothing precise enough for 100% confirmation.

The analysis of the flaperon will continue at the specialized lab near Toulouse, in southwestern France.

“The experts are carrying out their work promptly in order to provide complete and reliable information as soon as possible to the victims’ families,” Mackowiak said.

But he cautioned that he was unable to specify when the results of the ongoing analysis would be announced.

“On a forensic investigation, which is what’s going on in France, you don’t use the process of elimination and say, well, it must be a flap,” said David Soucie, a CNN aviation analyst. “You have to have forensic proof, which is the samples from the paint, from the metal, to tie it specifically to the aircraft. That’s what they’re waiting for, and that’s what they mean by 100% conclusive.”

Experts have said that investigators may be able to glean clues from the wing debris about Flight 370’s final moments before it is believed to have gone down in the southern Indian Ocean. Did it break apart in midair or hit the water intact?

But neither the French nor the Malaysians made any reference to that aspect of the analysis.

Australian Prime Minister Tony Abbott, whose country is leading the underwater search for the remains of MH370, said Thursday that the Reunion Island discovery “suggests that for the first time we might be a little bit closer to solving this baffling mystery.”

The remnants of a suitcase that were found near the flaperon on Reunion are being sent to a different French lab for examination, Mackowiak said.

Some relatives of those who were aboard the lost aircraft expressed bewilderment and frustration over the announcements.

“I was left somewhat confused and, frankly, a little angry and dismayed,” said K.S. Narendran, whose wife was one of the passengers.

“I didn’t hear facts. I didn’t hear the basics. I heard nothing,” he said, “and so it leaves me wondering whether there is a foregone conclusion and everyone is racing for the finish.”

The families of Chinese passengers, the most numerous nationality on the flight, were also unsatisfied.

“I don’t believe this. I don’t!” said Xu Jinghong, whose mother was on board Flight 370. “I am furious and I think this announcement is very irresponsible.”

Mary Schiavo, a CNN aviation analyst, said the families appeared to be stuck in the middle of a “tug of war among nations.”

But Sondra Wood, whose son Philip was on the airliner, said she feels the announcement “is the beginning of closure.”

“We know that it’s definitely in the ocean. It may be a journey or we may never find the plane, but at this point at least we don’t have to wonder, or guess, or fruitlessly hope that they could still be alive,” she told CNN’s “The Lead with Jake Tapper.”

Officials on Reunion are continuing to comb the island’s shores for more possible debris from Flight 370.

But it’s anyone’s guess whether many items from the aircraft are likely to find their way there.

“The problem is the only way for parts of the plane to get to Reunion Island is to continue to float,” Schiavo said. “The flaperon could because there are spaces inside that part that could allow air to be trapped and for it to float.”

But a lot of other potentially buoyant objects — like tennis shoes, bits of luggage and items from the plane’s galley — are likely to have become waterlogged and sunk after long months out in the ocean, she said.

The bigger, heavier parts of the aircraft are likely to have gone down into the depths very quickly.

The Australian-led underwater search for the wreckage and the remains of the people on board is taking place in a large, remote area of the southern Indian Ocean.

Locating the plane’s cockpit voice recorder and flight data recorder and cockpit is likely to be investigators’ best hope of unlocking the mystery of why it flew dramatically off course and lost communication with air traffic control.

“The finding of wreckage on La Reunion is consistent with our current search area,” Australian Deputy Prime Minister Warren Truss said Thursday. “For this reason thorough and methodical search efforts will continue in the defined search area.”

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