The piece of a plane wing that washed up on an island in the Indian Ocean, he announced, was indeed part of missing Malaysia Airlines Flight 370.

"It is my hope that this confirmation, however tragic and painful," Prime Minister Najib Razak said, "will at least bring certainty to the families and loved ones of the 239 people on board MH370."

But a top French prosecutor was slightly less definite when he stepped up to a podium in his country an hour later.

There are "very strong presumptions" that the part belongs to the missing Boeing 777, Paris Deputy Prosecutor Serge Mackowiak said, adding that more testing will be done to prove it conclusively.

France, which already had opened a criminal investigation into the plane's disappearance, has been drawn deeper into the matter after the plane part's discovery last month on Reunion island, a remote part of its overseas territory. Investigators at a specialized laboratory in Toulouse are examining it.

Even with tests ongoing, analysts said the Malaysian government's highly anticipated announcement marks a key step in the investigation into what happened to the plane.

"I was left somewhat confused and, frankly, a little angry and dismayed," said K.S. Narendran, whose wife was one of the passengers

Authorities announced their conclusions, Narendran said, without detailing their findings.

"I didn't hear facts. I didn't hear the basics. I heard nothing," he said, "and so it leaves me wondering whether there is a foregone conclusion and everyone is racing for the finish."

Investigators analyze debris

When the debris -- a part of a wing known as a flaperon -- washed up July 29 on Reunion island, its discovery was considered possibly the first physical evidence that might help shed light on a mystery that has vexed even the most seasoned aviation experts: How could a commercial airplane just vanish?

Photos: MH370 debris discovered on Reunion Island Photos: MH370 debris discovered on Reunion Island Debris discovered on the island of Reunion, a French territory in the Indian Ocean, was confirmed to be from Malaysia Airlines Flight 370, Malaysian Prime Minister Najib Razak said August 5. The plane disappeared in March 2014 with 239 people on board. Hide Caption 1 of 5 Photos: MH370 debris discovered on Reunion Island French police officers carry the plane debris on July 29. Experts say the metallic object may be a piece of a moving wing surface, known as a flaperon, from a Boeing 777. Hide Caption 2 of 5 Photos: MH370 debris discovered on Reunion Island Police officers inspect debris on July 29. Hide Caption 3 of 5 Photos: MH370 debris discovered on Reunion Island Another piece of debris resembles remnants of a suitcase. Hide Caption 4 of 5 Photos: MH370 debris discovered on Reunion Island Police carry debris on July 29. Hide Caption 5 of 5

On Wednesday, investigators met at a specialized laboratory near Toulouse to begin examining the part. Their work took hours, and Najib made the announcement very early Thursday, Kuala Lumpur time, 515 days since the flight bound for Beijing from the Malaysian capital disappeared with 239 people aboard.

Shortly before he spoke, Malaysia Airlines sent a message to victims' families saying a "major announcement" that the flaperon was from the missing plane was imminent.

"This has been confirmed jointly by the French Authorities, Bureau d'Enquetes et d'Analyses pour la Securites de I'Aviation Civile (BEA), the Malaysian Investigation Team, Technical Representative from PRC and Australian Transportation Safety Bureau (ATSB)," the airline's statement said.

The French prosecutor, who's involved in a criminal probe that country launched because four French nationals were aboard the flight, said the flaperon matches a Boeing 777, and the characteristics of the part match the technical specifications provided by Malaysia Airlines for that part of the missing aircraft.

But he phrased his assessment differently from the Malaysian Prime Minister, saying the analysis of the flaperon would continue in order to provide "complete and reliable information."

It sounded "less certain," said Mary Schiavo, a CNN aviation analyst and former inspector general for the U.S. Department of Transportation.

"Really, we didn't get too much more than what Boeing already told us from looking at the pictures," she said. "So I was actually a little disappointed, thinking what the families must think on hearing that."

In a "tug of war among nations," she said, the passengers' families seem to be stuck in the middle.

Photos: Remembering the passengers of MH370 Photos: Remembering the passengers of MH370 There is still no way to know for sure why Flight MH370 ended, but we are learning more about the lives of those on board. CNN is remembering them through snapshots shared with us. Hide Caption 1 of 12 Photos: Remembering the passengers of MH370 Rodney and Mary Burrows were looking forward to becoming first-time grandparents after their return home to Australia. Hide Caption 2 of 12 Photos: Remembering the passengers of MH370 Australians Catherine and Robert Lawton were traveling with friends on vacation when the flight disappeared. Hide Caption 3 of 12 Photos: Remembering the passengers of MH370 Paul Weeks was traveling to Mongolia for a new job as an engineer. His wife says Paul left behind his watch and his wedding ring before the trip, in case anything happened to him while he was away. Anderson spoke with Paul's brother & sister who said they are coping by spending time together as a family. Hide Caption 4 of 12 Photos: Remembering the passengers of MH370 Chandrika Sharma, left, was on Flight 370; her daughter Meghna and husband K.S. Narendran wait patiently, trying to manage their anxiety and longing for her return. Hide Caption 5 of 12 Photos: Remembering the passengers of MH370 Muktesh Mukherjee and Xiaomo Bai had been vacationing in Vietnam and were on their way home to their two young sons in Beijing. Hide Caption 6 of 12 Photos: Remembering the passengers of MH370 76-year-old Liu Rusheng, an accomplished calligrapher and one of the oldest passengers on the flight, was in Malaysia to attend an art exhibition with his wife. Hide Caption 7 of 12 Photos: Remembering the passengers of MH370 Teens Hadrien Wattrelos and Zhao Yan are shown in a photo on Wattrelos' Facebook page. The photo is captioned, simply, "I love you," in French. Hide Caption 8 of 12 Photos: Remembering the passengers of MH370 Firman Chandra Siregar, 24, studied electrical engineering in Indonesia and was on his way to Beijing on board Flight 370 to start a new job at an oil company. Hide Caption 9 of 12 Photos: Remembering the passengers of MH370 Patrick Francis Gomes, center, was the in-flight supervisor for the missing plane. His daughter describes him as a quiet person with a sense of humor. Hide Caption 10 of 12 Photos: Remembering the passengers of MH370 Ch'ng Mei Ling, a Malaysian citizen who lives in Pennsylvania, is a process engineer at a chemical company. Hide Caption 11 of 12 Photos: Remembering the passengers of MH370 We do not have photos of all 239 passengers, but we wanted to remember that there are loved ones around the world missing them right now. View CNN's complete coverage of Flight 370. Hide Caption 12 of 12

Relatives of those on board have said real closure won't come until their family members' remains have been recovered and the truth about what happened to the plane is established.

Progress on those fronts is unlikely to be made unless the Australian-led underwater hunt locates the aircraft's wreckage and flight recorders somewhere in the huge southern Indian Ocean search area, which covers and area bigger than the U.S. state of Pennsylvania.

Global effort to solve mystery

Malaysia, the country where MH370 began its journey and whose flag it was carrying, is in charge of the overall investigations.

But there are a number of other countries involved.

French authorities have also opened their own criminal investigation last year into possible manslaughter and hijacking in the loss of MH370.

Officials from Australia earlier said they thought it was likely that the Boeing wing component was from MH370 -- no other 777 aircraft was believed to have gone missing in the Indian Ocean.

Australia is overseeing the underwater search for the wreckage because the plane is believed to have gone down far off its western coast, in a remote part of the southern Indian Ocean.

China, which had the largest number of citizens on the plane, has been involved in decisions about the search for the plane.

Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

U.S. and British government agencies -- as well as experts from Boeing and the satellite company Inmarsat -- have contributed to the investigations.

The Toulouse lab previously examined wreckage from Air France Flight 447, a passenger jet that went down in the Atlantic Ocean en route from Rio de Janeiro to Paris in 2009.

The remnants of a suitcase found on Reunion the day after the flaperon was discovered have been sent to a lab outside Paris for analysis.

Debris won't change underwater search

The wing part found on Reunion won't prompt a rethink of the search area, Australian officials say.

JUST WATCHED From disappearance to debris: CNN's coverage of MH370 Replay More Videos ... MUST WATCH From disappearance to debris: CNN's coverage of MH370 03:16

"Because of the turbulent nature of the ocean, and the uncertainties of the modeling, it is impossible to use the La Reunion finding to refine or shift the search area," the Australian Transport Safety Bureau said in a report published Tuesday, citing the country's national science agency.

That the debris drifted thousands of miles to the west of the underwater search area is consistent with ocean drift models, Australian officials say.

The ATSB report admitted, though, that an earlier prediction that some debris from MH370 could wash up in July 2014 on the shores of the Indonesian island of Sumatra, north of where the aircraft is calculated to have entered the ocean, was incorrect because of an error in the use of wind data.

Searches are taking place on Reunion for more possible debris from MH370. But Australian Deputy Prime Minister Warren Truss has said he's doubtful many pieces are likely to turn up.

"Reunion Island is a pretty small speck in a giant Indian Ocean," he told The Wall Street Journal. "Most pieces that were even floating by the time they got to this area would simply float past."