KUALA LUMPUR, Malaysia (AP) — The confusion and frustration that punctuated the investigation into the disappearance of Malaysia Airlines Flight 370 erupted anew Thursday after officials offered conflicting levels of confidence on whether a piece of a wing found washed up on an Indian Ocean island last week came from the vanished plane.

Malaysian Prime Minister Najib Razak's announcement in the pre-dawn hours of Thursday that the part, known as a flaperon, did indeed come from the doomed aircraft was at odds with the message from authorities in France, the U.S. and Australia, who have stopped short of full confirmation.

The conflicting comments raised questions about why Malaysia had seemingly gone rogue with its announcement and infuriated many families of those on board the plane, who have waited more than 500 days for concrete clues into the fates of their loved ones. Dai Shuqin, the sister of one of the passengers, was among about a dozen Chinese relatives who held a demonstration outside Malaysia Airlines' offices in Beijing.

"France is being cautious about it, but Malaysia is desperate to put an end to this case and run away from all responsibilities," she said.

The disappearance of the Boeing 777 jetliner while on a flight from Kuala Lumpur to Beijing on March 8, 2014, has been one of the biggest mysteries in aviation history. Officials believe it crashed in the southern Indian Ocean, killing all 239 people on board, but the reasons why remain elusive.

"It is with a very heavy heart that I must tell you that an international team of experts has conclusively confirmed that the aircraft debris found on Reunion Island is indeed MH370," Prime Minister Najib Razak told reporters. The French territory is thousands of kilometers (miles) west of the area being searched for wreckage from the flight.

But at a news conference in Paris, Deputy Prosecutor Serge Mackowiak declined to confirm the debris belonged to Flight 370, though he said there were strong indications that it did.

"The very strong conjectures are to be confirmed by complementary analysis that will begin tomorrow morning," Mackowiak said. "The experts are conducting their work as fast as they can in order to give complete and reliable information as quickly as possible."

The caution was typical of how France carries out air crash investigations. The French agency that usually handles such probes, known as the BEA, can take months if not years to lay out exhaustive conclusions in reports that last hundreds of pages. During the inquiries, they only rarely offer interim assessments and even more rarely comment.

The Australian government, which leads the seabed search for wreckage west of Australia, was also less certain than Malaysia, saying only that "based on high probability, it is MH370."

Publicly, Australian officials withheld criticism of Najib's announcement, with Australian Transport Minister Warren Truss saying Australia respected Malaysia's right to make that call, given it is the government in charge of the investigation.

"Of course there is still some i's not dotted and t's not crossed. There is still a very small element of doubt," Truss said. "The French are being more cautious. But from the Malaysians, the flag carrier of the aircraft, they are now satisfied that the body of evidence is sufficient to say that this is wreckage from MH370."

Privately, however, there were questions about why Najib had moved forward with the statement before all countries had agreed. An Australian government official, speaking on condition of anonymity because he wasn't authorized to comment publicly, said Malaysia wasn't supposed to make the announcement, and had gone out on its own making a conclusive statement before getting the evidence to back it up.

Many families of those on board, who have waited nearly 17 months for tangible evidence, were fed up with the mixed messages.

"Why the hell do you have one confirm and one not?" said Sara Weeks, the sister of New Zealander Paul Weeks, who was on board. "Why not wait and get everybody on the same page so the families don't need to go through this turmoil?"

Ross Tapsell, a Malaysia expert at the Australian National University, said he suspected Najib had snatched an opportunity to distract Malaysians from a corruption scandal that threatens his power. On Monday, Malaysia's anti-corruption agency said that $700 million in Prime Minister Najib Razak's personal bank accounts came from donations, not from a debt-ridden state investment fund.

"My first gut feeling is that Najib is trying to take the attention away from his own political scandal," Tapsell said. "He's under so much political pressure at the moment. ... I presume if he can try to move the conversation back to the MH370 stuff, it's in his interests to do so."

Najib would also want to stamp his authority on the search, which Malaysia oversees, Tapsell said, rather than allow France to dominate attention through its leading role in examining the wing fragment.

Damien Kingsbury, a South Asia expert from Australia's Deakin University, said Malaysia tends to be a combative nation that does not have a cozy relationship with any country; it is therefore not surprising that it opted to act unilaterally with the announcement.

"Malaysia's identity is promoted in a fairly forthright manner, so it's not interested particularly in the sensitivities of other countries unless they have some advantage to offer — and France is probably not seen in that light," he said.

A U.S. official familiar with the investigation said the flaperon clearly is from a Boeing 777. However, a team of experts in France examining the part hadn't yet been able to find anything linking it specifically to the missing plane, the official said, speaking on condition of anonymity because there was no authorization to talk publicly about the case.

With no other 777s or flaperons known to be missing, it makes sense that the part comes from Flight 370, but the U.S. and Boeing team members are merely trying "to be precise," the official said.

Australia, which has sent an official to France to help examine the flaperon, has said the find will not affect its sonar search of a 120,000-square-kilometer (46,000-square-mile) expanse of seabed more than 4,000 kilometers (2,500 miles) east of Reunion Island.

That search, which began in October, has covered almost half that area without finding any clues.

"The fact that this wreckage does now look very much like it is from MH370 does seem to confirm that it went down in the Indian Ocean, it does seem very consistent with the search pattern that we've been using for the last few months," Australian Prime Minister Tony Abbott told Melbourne Radio 3AW. "Let's hope we can turn something up."

Intact and encrusted with barnacles, the flaperon was found on a beach and sent to France for scrutiny by the BEA and members from its Malaysian and Australian counterparts.

Analysts say the investigators will examine the metal with high-powered microscopes to gain insight into what caused the plane to go down. It is also not known why Flight 370 — less than an hour into the journey — turned back from its original flight path and headed in an opposite direction before turning left and flying south over the Indian Ocean for hours.

Malaysia Airlines said in a statement that the finding was "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."

Analysts have said an examination of the wing part could indicate what kind of stress the plane was under when it hit the water. But it won't fully solve the mystery of why the plane disappeared, nor will it help pinpoint where the plane crashed.

A six-week air and sea search covering 4.6 million square kilometers (1.8 million square miles) of the southern Indian Ocean surface last year failed to find any trace of the jetliner. The Reunion Island debris would support the working theory that the jet went down in the Indian Ocean and the debris was carried by the current, which moves counterclockwise.

Malaysian officials have said the plane's movements were consistent with deliberate actions by someone on the plane, suggesting someone in the cockpit intentionally flew the aircraft off course.

Since last year, Australian officials who are leading the search effort have operated on the theory that the plane flew on autopilot for hours before running out of fuel and crashing into the ocean. Investigators settled on that scenario after analyzing data exchanged between the plane and a satellite, which showed the jetliner took a straight path across the ocean.

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Associated Press writers Kristen Gelineau in Sydney, Paul Joshua in Kuala Lumpur, Thomas Adamson in Paris, Joan Lowy in Washington, Rod McGuirk in Canberra, Australia, Nick Perry in Wellington, New Zealand, and Isolda Morillo and Aritz Parra in Beijing contributed to this report.