It's the biggest lead in the 16-month-old mystery of the disappearance of Malaysia Airlines Flight 370. Aviation experts say the debris is consistent with that from a Boeing 777, and MH370 is only Boeing 777 that has gone missing over water.
"If it is indeed part of a 777, then we're pretty clear that it would be then also part of MH370," said David Soucie, a CNN aviation analyst and safety and accident investigator.
On Thursday, Boeing investigators had a high level of confidence that the debris found on Reunion Island came from a 777 because of photos that have been analyzed and because of a component number on the debris that "corresponds to a 777 part," according to a source close to the investigation.
What was found, and why is it so intriguing?
Despite Boeing's confidence, investigators from the United States want to see the part up close to make a final determination.
Boeing and U.S. investigators from the National Transportation Safety Board will be given access to the debris once international investigators get it to a lab for examination.
Experts say the metallic debris may be a piece of a moving wing surface known as a flaperon from a Boeing 777.
According to the Aviation Safety Network, which tracks aviation accidents, there have been only five 777 accidents in which the aircraft were destroyed. Three happened at airports in London, Cairo and San Francisco. MH17 was shot down over Ukraine, and MH370 disappeared over water.
What are searchers doing now?
Investigators continue to arrive at the area where the debris was found. A police helicopter is flying along Reunion's shoreline, according to a CNN team there. Police are also encouraging locals to look for debris around the beach.
Malaysian Prime Minister Najib Razak says the debris will be flown to Toulouse, France, and given to BEA, the French aviation safety bureau.
Madame Thibault-Le Cuivre, the spokesperson for the chief prosecutor on Reunion Island, said French police will oversee the transport of the debris.
How far away was debris found, and when and where did 370 go missing?
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.
Malaysian authorities revealed later that military radar had tracked the plane as it turned back to the west, flew across the Malaysian Peninsula, and up the Strait of Malacca, before flying out of radar range at 2.14 a.m. and vanishing once again.
How are the families doing?
Chinese relatives of those on board issued a joint statement calling for more information and for the search not to be called off until the plane and passengers' final resting place is found.
Sarah Bajc's partner Philip Wood was on the plane. She told CNN on Thursday: "On the one hand, I really don't want it to be part" of finding MH370 debris, so that she could "still keep hope. ..."
But, "on the other hand, we're all pretty exhausted ... and need a little closure," she said.
Her voice cracking, a very emotional Bajc said she has consistently distrusted the Malaysian government and that nation's aviation authorities because she feels the investigation into the plane's disappearance has been handled poorly.
She takes some comfort in the debris being found in French territory and feels the French can handle the case better.
What are the theories about the plane?
In the absence of any firm answers, competing theories have surfaced
as to what led to the plane's mysterious disappearance along a circuitous flight route.
One theory is that one of the pilots may have incapacitated the other, then deliberately taken the plane down in an act of pilot murder-suicide -- a nightmarish scenario that would subsequently become a reality in the Germanwings crash of March 2015.
Some have suggested that a guest in the cockpit could have commandeered or hijacked the plane, although the lack of any claim of responsibility makes this less likely. Others say a more conventional explanation, such as mechanical failure, is more likely.
What are theories about where it is?
Initially, planes and ships from a number of countries searched for MH370 in areas of the South China Sea, south of Vietnam and around the Malaysian Peninsula.
The search area was then expanded into the Andaman Sea, as authorities revealed that the plane could have flown for a number of hours after last contact.
A week after the disappearance, Razak, the Malaysian Prime Minister, said the aircraft was deliberately diverted and had continued flying for more than six hours after losing contact.
Conflicting theories emerged about the plane's flight path, with speculation about whether it took a "northern arc" or a "southern arc," and the search area in the southern Indian Ocean changed several times.
After it was determined, through an analysis of "handshakes" between the plane and an Inmarsat telecommunications satellite, that MH370 had eventually turned and flew south for hours, searchers focused their attention on a swath of the Indian Ocean about 2,000 kilometers (1,243 miles) off Australia's west coast known as the "seventh arc."
"All the available data indicates the aircraft entered the sea close to a long but narrow arc of the southern Indian Ocean," the Australian Transport Safety Bureau says on its website.
Australia has taken the lead in the search and recovery operation in support of the Malaysian accident investigation since March 31, 2014.
Hopes were raised several weeks after the aircraft's disappearance, when a commercial satellite and then a Chinese satellite made sightings of large objects in the southern Indian Ocean that Australian and Chinese authorities believed could be debris from the aircraft.
The sightings led nowhere.
Not long after, there was optimism that searchers had made a breakthrough when four acoustic "pings" were detected in an area of the southern Indian Ocean on April 5 and 8.
Officials expressed "cautious optimism" about the faint noises. But after seven weeks focusing their efforts on the area where the sounds were heard, searchers discounted the notion
that the pings had come from the missing plane's "black boxes," or voice and flight data recorders.