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The airplane fragment that washed up on an island last week was a piece of Malaysia Airlines Flight 370, the Malaysian prime minister confirmed Wednesday — the first definitive physical clue to the greatest mystery in modern aviation.

“I hope 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,” said a somber Prime Minister Najib Razak. “They have our deepest sympathy and prayers.”

The fragment — a 6-foot-long, barnacle-encrusted wing flap — was discovered on July 29 by a crew cleaning the beach on Reunion Island, a French territory in the Indian Ocean off the southern tip of Africa.

Investigators had already determined that it came from a Boeing 777, and Flight 370 was the only plane of that model missing in the world.

But the confirmation on Wednesday provided the first tangible evidence of what became of the plane after it disappeared from radar on March 8, 2014, with 227 passengers and 12 crew on board.

Najib delivered the news on national television in Malaysia.

“Today, 515 days since the plane disappeared,” he said, “it is with a very heart that I must tell you that an international team of experts have conclusively confirmed that the aircraft debris found on Reunion Island is indeed from MH370.”

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Investigators had analyzed the wing fragment, known as a flaperon, at a laboratory in Toulouse, France. French investigators were also analyzing a shredded suitcase found on Reunion Island, but there was no immediate word on a conclusion about that piece of debris.

In a statement, Malaysia Airlines said that the confirmation of the wing fragment was reached jointly by French, Malaysian and Australian investigators.

“Family members of passengers and crew have already been informed and we extend our deepest sympathies to those affected,” the airline said.

“This is indeed a major breakthrough for us in resolving the disappearance of MH370. We expect and hope that there would be more objects to be found which would be able to help resolve this mystery.”

Serge Mackowiak, a deputy prosecutor in France, used slightly less conclusive language than the prime minister and the airline: He said that investigators “can very strongly presume” that the wing flap came from Flight 370.

He said that additional experts would be able to confirm the finding by Thursday.

Sarah Bajc, whose boyfriend Philip Wood was on the flight, said in a statement to NBC News: "Now families finally have the chance to grieve, though this doesn't solve the mystery or hold anyone accountable. Both of those things still have to happen."

Li Li, whose father was aboard MH370, told NBC News she was in disbelief.

"I wonder how credible the news is. How could they ever find that out?" she said.

Steve Wang, leader of the MH370 family committee in China whose mother is on the plane, also expressed some doubt in the findings.

"I am not completely convinced that the debris belongs to the missing plane," Wang told CNBC. "The French side's remarks are (only) strong supposition. I don't understand why — once again — the Malaysian side rushed to a conclusion and made such an announcement. Even if the debris ultimately belongs to MH370, this is only the beginning of the investigation. There are many questions remaining. Where is the main body of the plane? What on earth happened to it? Who caused it? Who should be held responsible? Who should be punished? We families demand the answers. This is just the beginning and it will be a long process to figure it all out."

The Chinese Foreign Ministry issued a statement offering condolences to the families of the Chinese nationals who were on board the flight. "We demand the Malaysian side carry out relevant promises, continue their investigation into the cause of the plane crash, go all-out to handle the issues arising from the incident and earnestly safeguard the legal rights and interests of the families of the passengers,” the statement said.

Experts have cautioned that the wing flap could provide a clue about the moment when plane hit the water, but will not resolve the basic question of what went wrong on its journey from Kuala Lumpur, Malaysia, to Beijing.

It is also unlikely to pinpoint the location of the body of the airplane, and thus the flight data recorder and cockpit voice recorder. For months, international teams have been searching a swath of the ocean depths much closer to Australia.

Oceanographers say that Indian Ocean currents could easily have carried debris counterclockwise from the search zone toward the coast of Africa in the 17 months since the plane was lost.