The airline says the pilots did not send any distress signals, suggesting a sudden and possibly catastrophic incident. Speculation has ranged widely about possible causes, including pilot error, plane malfunction, hijacking and terrorism.
News that two of the passengers were traveling with stolen passports immediately fueled speculation of foul play. However, Malaysian police chief Khalid Abu Bakar told a news conference Tuesday that investigators had determined one was a 19-year-old Iranian, Pouria Nourmohammadi Mehrdad, and that it seemed likely that 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 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."
Khalid said the 19-year-old man's mother was waiting for him in Frankfurt and had been in contact with police. He said she contacted Malaysian authorities to inform them of her concern when her son didn't get in touch with her.
He also said there was no truth to a statement by at least one other government official that five passengers had checked in for the flight but never boarded the airplane.
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.
Authorities have said the plane may have attempted to turn back toward Kuala Lumpur.
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.
Malaysia Airlines said search and rescue teams have expanded the scope beyond the flight path to the Straits of Malacca between Malaysia's western coast and Indonesia's Sumatra island — the opposite side of Malaysia from its last known location.
An earlier statement said the western coast of Malaysia was "now the focus," but the airline subsequently said that phrase was an oversight.
"The search is on both sides," Civil aviation chief Azharuddin Abdul Rahman said.
The search currently includes nine aircraft and 24 ships from nine countries that have been scouring the Gulf of Thailand on the eastern side of Malaysia. Land areas also are being searched.
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.