Only one Boeing 777 has ever disappeared over water: Malaysia Airlines Flight 370.

American investigators tell The New York Times that a part of an airplane found washed up on a beach on the Indian Ocean island of Réunion is from a Boeing 777. “A person familiar with the matter” tells Reuters the part was almost certainly from a 777.

This can only be MH370.

After a fruitless 17-month search, the discovery could be the beginning of solving the world’s greatest aviation mystery—the first piece of physical evidence that investigators can examine. A few experts still have reservations about the certainty that the debris is from the 777, but most do not.

The piece of debris is part of a wing and probably a control surface from that wing. Control surfaces are among the most likely part of airplane wreckage to survive for a long while as flotsam. The most buoyant parts of an airplane are the lightest parts of the wings, like the flaps and ailerons that are hinged to the rear of the wings and are easily separated by impact on water or land (and for the same reason the horizontal and vertical control surfaces of the tail).

The dimensions of the piece that washed up on Réunion, 9 feet by 3 feet, are consistent with the size of several pieces of a 777’s control surfaces, possibly what is called a flaperon on the inner rear section of the wing.

As encouraging as this discovery is, it does raise important caveats.

Why would only one fragment turn up? Airplane wreckage is normally found in clusters.

That means an air-and-sea search of the area east of Réunion should be launched in the belief that other debris will be visible in the area—the more physical evidence gathered, the better.

Why is it so far from where the experts involved in the search had predicted the wreckage would show up? The Australian authorities leading the search for Flight 370 said last fall that they were working on a drift model to accurately plot where wreckage might first show up. Their prediction: the southern coast of Sumatra—4,500 miles away from Réunion.

The Australians said they would release the results of this drift modeling early this year but they never have. Two months ago they told The Daily Beast: “The work, once finalized, will be released.”

If a piece of the 777 has turned up so far away from the predicted landfall, does it mean the Australian search has been in the wrong place all along?

Not necessarily. Ocean currents are very complex—the main part of Flight 370 could still be where the Australians are searching, 1,000 miles west of the Australian coast on the deep ocean floor.

Warren Truss, Australia’s deputy prime minister, issued a statement saying that if the wreckage is identified as being from MH370, “it would be consistent with other analysis and modeling that the resting place of the aircraft is in the southern Indian Ocean.”

Brett Heffernan, a spokesman for the deputy prime minister, told The Daily Beast that the discovery does not diminish their confidence that the area of the Indian Ocean being searched is the right area.

There is no “conveyor belt” certainty about currents in the Indian Ocean. The main influence on currents is the giant rotating “gyre,” one of five in the world’s oceans, but there are many other patterns that could play a part, including winds and weather.

The heaviest part of the airplane and most likely to sink to the floor immediately are the engines, designed to shear away from the wings on impact. This would be followed by what is called the wing box, where the wings join the fuselage, which also contains the central fuel tank and the main landing gear. Because of its size and mass, this part of the airplane creates the optimum target for undersea searching by sonar.

Nonetheless, any piece of wreckage, no matter from what part of the airplane, has a story to tell that investigators will immediately look for. After 17 months in seawater, the piece of the wing looks remarkably intact, apart from a crust of barnacles.

Investigators will first be able to assess with some accuracy the force that tore the piece from the rest of the wing, giving a picture of how the 777 impacted the ocean. They will then be able to judge whether the airplane broke up in the air or only when it hit the water. They will also look for any signs of fire damage—unlikely.

The ultimate answer to what became of Flight 370, then, is still a long way off, but the first clue is with us.