KUALA LUMPUR, Malaysia >> Authorities are expanding their search for a missing Malaysia Airlines plane to the Malacca Strait, far from its last confirmed location, the airline said Tuesday, injecting new mystery into an investigation that so far has failed to come up with any answers.

More than three days after the Boeing 777 disappeared en route from Kuala Lumpur to Beijing, no trace of the plane has been found in waters between Malaysia and Vietnam that have been scoured by more than 40 planes and ships from at least 10 nations.

The plane, carrying 239 people, dropped off radar less than an hour into the flight without sending out a distress signal. Authorities have said it may have attempted to turn back to Kuala Lumpur, but they expressed surprise that it would do so without informing ground control.

Malaysia Airlines said in a statement that search and rescue teams expanded their scope to the Malacca Strait between Malaysia’s western coast and Indonesia’s Sumatra island — the opposite side of Malaysia from the plane’s last known location.

To reach the strait, a busy shipping lane, the plane would have had to cross over the country, presumably within the range of radar.

An earlier statement said the western coast of Malaysia was “now the focus,” but the airline subsequently said that phrase was an oversight. It didn’t elaborate. Civil aviation chief Azharuddin Abdul Rahman said the search remained “on both sides” of the country.

Also Tuesday, authorities said two people who boarded the flight using stolen passports were Iranians who had purchased tickets to Europe. Their appearance on the flight had raised speculation of a possible terrorist link.

Malaysian police chief Khalid Abu Bakar said investigators had determined one was a 19-year-old Iranian, Pouria Nourmohammadi Mehrdad, and that it seemed likely he was planning to migrate to Germany.

“We believe he is not likely to be a member of any terrorist group,” Khalid said.

Interpol identified the second man as Seyed Mohammed Reza Delavar, a 29-year-old Iranian, and released an image of the two boarding a plane at the same time. Interpol Secretary General Ronald K. Noble said the two men traveled to Malaysia on their Iranian passports, then apparently switched to their stolen Austrian and Italian documents.

He said speculation of terrorism appeared to be dying down “as the belief becomes more certain that these two individuals were probably not terrorists.” He appealed to the public for more information about the two.

The plane took off from Kuala Lumpur, on the western coast of Malaysia, early Saturday en route to Beijing.

It flew across Malaysia into the Gulf of Thailand at 35,000 feet (11,000 meters) and then disappeared from radar screens.

The hunt began on Saturday near the plane’s last known location. But with no debris found there, the search has been systematically expanded to include areas the plane could have reached with the fuel it had on board. That is a vast area in which to locate something as small as a piece of an aircraft.

China, where two-thirds of the passengers are from, urged Malaysian authorities on Tuesday to “speed up the efforts” to find the plane. It has sent four ships, with another four on the way.

A shopping mall in Beijing suspended advertising on its large outdoor LED screen to display a search timer — an image of an airplane along with a digital clock marking the time since contact with the flight was lost.

Assuming the plane crashed into the ocean or disintegrated in midair, there will likely still be debris floating in the ocean, but it may be widely spread out and much may have already sunk. In past disasters, it has taken days or longer to find wreckage.

The United States has sent two navy ships, at least one of which is equipped with helicopters, and a Navy P-3C Orion plane with sensors that can detect small debris in the water.

Vietnamese planes and ships are also taking part. Lt. Gen. Vo Van Tuan, deputy chief of staff of the Vietnamese People’s Army, said authorities on land had also been ordered to search for the plane, which could have crashed into mountains or uninhabited jungle. He said military units near the border with Laos and Cambodia had been instructed to search their regions also.

“So far we have found no signs … so we must widen our search,” he said.

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Minh reported from Hanoi, Vietnam. Associated Press writers Jim Gomez and Eileen Ng in Kuala Lumpur and Chris Bodeen in Beijing contributed to this report.