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.