Authorities said for the first time Saturday that Malaysia Airlines Flight 370 veered sharply off its flight plan because of “deliberate action by someone on the plane,” communicated with satellites for hours after it disappeared and might have ended up thousands of miles away.

Prime Minister Najib Razak stopped short of saying that the plane was hijacked. He said that the investigation would concentrate on both the passengers and the crew.

“I wish to be very clear,” he said. “We are still investigating all possibilities as to what caused MH370 to deviate from its original flight path.”

His disclosures, at a press briefing in the Malaysian capital of Kuala Lumpur, represented the first big break in the investigation since the plane disappeared a week ago, with 239 people on board, on an overnight flight to Beijing.

In Beijing, relatives of people who were on board the plane gathered to watch the prime minister speak and gasped at what he had to say. One woman whose husband was on the plane said that a hijacking might even be encouraging news, “because they could still be alive.”

But the prime minister’s remarks also underscored the challenge ahead — two enormous “corridors” of land and sea that must yet be searched.

As described by Razak, a northern search corridor stretches from the border of Kazakhstan and Turkmenistan to northern Thailand. That zone covers practically all of south Asia and includes Afghanistan, Pakistan and India.

A southern search corridor stretches from Indonesia into the vast southern reaches of the Indian Ocean.

Razak said that satellite information, confirmed only hours earlier, corroborated what was picked up on Malaysian military radar: After it lost contact with ground control, Flight 370 turned sharply to the west, back over land, and then northwest, toward the Indian Ocean.

The prime minister also revealed that the last communication between the plane and satellites was at 8:11 a.m. local time Saturday — more than five hours after the plane lost touch with controllers on the ground. That would have given it time to fly thousands of miles farther.

Razak said other countries in the two new search corridors will be asked to help. The search will be called off, he said, in the South China Sea — the water between Malaysia and Vietnam where Flight 370 was lost.

It remained unclear at what altitude the aircraft was traveling when it made the suspicious turns. U.S. government investigators discounted a report that the jet, a Boeing 777, climbed to 45,000 feet and then dived suddenly. The investigators said the data for altitude were unreliable.

Authorities said they would brief reporters later in the day. The prime minister did not take questions.

His appearance at the daily press briefing on the investigation, his first, underscored both the agony of the long wait for the families and the significance of his revelations. And he went out of his way to say that he understood it had been an excruciating time.

“No words can describe the pain they must be going through,” he said.

In the week since the jet vanished, hopes have been dashed again and again. Oil slicks turned out to contain no jet fuel, suspected debris was nothing more than floating trash, and theories were floated and discredited.

But the prime minister said that finding the plane was so important that Malaysia was willing to put its own national security second to the search. Thirteen countries, including the United States, are helping Malaysia, and a total of 43 ships and 58 aircraft are looking for the jet, a Boeing 777.

Razak described the investigation as having entered “a new phase.”

“We hope this new information brings us one step closer to finding the plane,” he said.

Based on the satellite information, Razak said, investigators can say “with a high degree of certainty” that the plane’s ACARS system, which transmits short messages to ground control, was turned off just before the plane crossed from the Malaysian peninsula into the South China Sea.

Shortly after that, he said, near the point where the plane would have been handed off from Malaysian to Vietnamese air traffic control, the plane’s transponder was turned off.

The prime minister did not go into detail about the satellite information. But on Friday, the satellite communications company Inmarsat said that its satellites picked up “routine, automated signals” from Flight 370 during its journey from Kuala Lumpur.

By default, the company’s satellites send a “ping” once an hour to devices registered with Inmarsat, and active devices send back a “ping” to the nearest satellite. Most wide-body jets carry Inmarsat equipment.

That information can be used to determine speed and altitude, and could be crunched to at least narrow the geographic range where a plane might be. Inmarsat said its information had been shared with Malaysian investigators.