PARIS — Investigators and the families of those who were aboard Malaysia Airlines Flight 370 now have what they have sought for more than a year — the first tangible trace of the vanished airplane.

There are “very strong presumptions” that the airplane part that washed ashore last week on the French island of Réunion in the western Indian Ocean came from the missing Boeing 777, an official said on Wednesday at a Paris news conference after experts inspected the object.

A few minutes before the news conference, Prime Minister Najib Razak of Malaysia went further, declaring that the object definitely came from the plane, which disappeared in March 2014 with 239 people aboard.

A person involved in the investigation said, however, that experts from Boeing and the National Transportation Safety Board who had seen the object, a piece of what is known as a flaperon, were not yet fully satisfied, and called for further analysis.