The news, based on fresh evidence gleaned from an unprecedented analysis of satellite data, meant it was all but impossible that any of the 239 passengers and crew on board the jetliner could have survived.
That realization may help bring some closure to families 17 days after their nightmare began when the Boeing 777 inexplicably disappeared from Asian skies during what was supposed to be a routine overnight flight from Malaysia's capital to Beijing on March 8. But the latest clue is also only a small step toward solving one of the greatest puzzles in aviation history.
With the location of Flight 370 itself still unknown - most likely somewhere at the bottom of the sea in a remote part of the southern Indian Ocean - profound questions remain unanswered about what brought down the aircraft and why.
And the grueling search for the wreckage and the plane's black boxes could take years. The task, involving a multinational force sweeping a vast region of ocean whose dark floor is up to 7,000 meters (23,000 feet) deep, has been daunting. So much so, that it is also possible that what is left of the plane may never be found.
In Beijing, family members who have followed every twist and turn in the search shrieked and sobbed uncontrollably when they heard the news. One woman collapsed and fell on her knees, crying "My son! My son!"
Dressed in a black suit, Malaysian Prime Minister Najib Razak somberly announced the news in an unexpected late night statement to reporters in Kuala Lumpur. The information, he said, was based on a study of data from a satellite that had received the final known signals from the plane as it tracked southward.
The data indicated that the jetliner flew "to a remote location, far from any possible landing sites," Najib said.
"It is therefore with deep sadness and regret that I must inform you that, according to this new data, Flight MH370 ended in the southern Indian Ocean."
In China's capital, family members had been called to a hotel near the airport to hear the announcement. Afterward, they filed out of a conference room in heart-wrenching grief.
Medical teams arrived with several stretchers and at least one elderly man was carried out of the conference room on one of them, his faced covered by a jacket. Minutes later a middle-aged woman was taken out on another, her face ashen and her eyes blank and expressionless.
Nan Jinyan, whose brother-in-law Yan Ling was aboard the flight, said she had been prepared for the worst but the announcement was still "a blow to us, and it is beyond description."
In Kuala Lumpur, family members also broke down in sobs.
Selamat Omar, the father of a 29-year-old aviation engineer who was on the flight, said: "We accept the news of the tragedy. It is fate."
In a statement to the families, Malaysia Airlines said: "We know there are no words that we or anyone else can say which can ease your pain."
The airline said it would transport families to the western Australian city of Perth if parts of the plane are found and brought to a military base there now being used by search planes. It said the "ongoing multinational search operation will continue as we seek answers to the questions which remain."
The plane's disappearance has baffled investigators, who have yet to rule out mechanical or electrical failure, hijacking, sabotage, terrorism or issues related to the mental health of the pilots or someone else on board.
Malaysian authorities have said that evidence so far suggests the plane was deliberately turned back across Malaysia to the Strait of Malacca, with its communications systems disabled. They are unsure what happened next.
Earlier Monday, Malaysia's police chief, Inspector General Khalid Abu Bakar, reiterated that all the passengers had been cleared of suspicion. But he said the pilots and crew were still being investigated and declined to comment on whether officials had recovered files that were deleted a month earlier from the home flight simulator of the chief pilot.
Officials have said the plane automatically sent a brief signal - a "ping" - every hour to a satellite belonging to Inmarsat, a British company, even after other communication systems on the jetliner shut down.
The pings did not include any location information, but an initial analysis showed that the location of the last ping was probably along one of two vast arcs running north and south.
Najib said Inmarsat did further calculations "using a type of analysis never before used in an investigation of this sort," and had concluded that the plane's last position was "in the middle of the Indian Ocean, west of Perth."
He gave no indication of exactly where in the Indian Ocean the plane was last heard from, but searchers have sighted possible debris in an area about 2,000 kilometers (1,240 miles) southwest of Perth, and Najib said more details would be released Tuesday.
Search teams from 26 nations have pored over radar data and scoured a wide swath of Asia for weeks with advanced aircraft and ships, but no confirmed wreckage has been found.
The hunt is now considered a race against time because of the battery life of the "pinger" in the black box is running out and may have less than two weeks left.
On Monday, an Australian navy support vessel, the Ocean Shield, headed toward the search zone and was expected to arrive in three or four days, a defense official said. The ship is equipped with acoustic detection equipment that can search for the black box.
The U.S. Pacific Command said it is also sending a black box locator in case a debris field is located, one that can be pulled behind a vessel at slow speeds and could hear the pinger down to a depth of about 20,000 feet (6,100 meters), Cmdr. Chris Budde, a U.S. 7th Fleet operations officer, said in a statement.
The deployment is part of "a prudent effort to pre-position equipment and trained personnel closer to the search area," he said.
U.S. Deputy National Security Adviser Ben Rhodes on Monday stopped short of saying the U.S. had independent confirmation of the status of the missing airliner. He noted the conclusion of Malaysian authorities that the Boeing 777 had plunged into the Indian Ocean and said the U.S., which has been assisting the search effort, was focused on that southern corridor of the ocean.
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