French authorities carried a large piece of plane debris found on the Indian Ocean island of Réunion, July 29, 2015. Reuters/Zinfos974/Prisca Bigot

Tests in France on a wing part -- possibly from the missing Malaysia Airlines Flight 370 -- have been delayed because several experts involved in the testing have not yet returned from vacation, the French newspaper Le Monde reported Wednesday. The wing part, called a flaperon, was discovered in late July on the Indian Ocean island of Réunion, and was "confirmed" as part of Flight 370 in early August by the Malaysian prime minister.

But authorities outside Malaysia -- including at Boeing, the U.S. maker of the missing plane -- doubted the Malaysian confirmation and pursued further testing on the flaperon. The plane in question disappeared in March 2014 while traveling from Kuala Lumpur, Malaysia, to Beijing, with 239 passengers and crew aboard.

The flaperon was taken to France and has been undergoing tests for the past two weeks. But this week French researchers were awaiting the return of a Spanish subcontractor who could reportedly trace the origins of the flaperon's sale and say with certainty whether the part is from the missing plane -- or from another Boeing 777 aircraft.

Authorities have not yet been able to determine why the plane went off the radar system, or where it went, in March 2014. Most experts have said the plane likely crashed somewhere in the Indian Ocean, but the plane was never found despite a massive, multinational search. Reports Wednesday suggested that the search could resume, using new sonar equipment.

The debris found on Réunion, a French territory east of Madagascar, is the strongest piece of evidence to surface so far in the Flight 370 investigation. Other items found on Réunion days later included seat cushions and window panes, and still more debris was found in the Maldives, but some experts doubted that plane debris could reach both of those locations. Despite more than a year of searching, no significant plane parts have been found except for the flaperon.

Researchers testing the debris have said that they would need to see an identification plate somewhere on or inside the flaperon to know with certainty that it was from Flight 370 or from another plane. The first phase of testing, which took place in a laboratory in Toulouse in southern France, was completed Aug. 19, and the results from that test have not reached any conclusions.