Nearly three days after the Boeing 777 with 239 people on board disappeared en route from Kuala Lumpur to Beijing, no debris has been seen in Southeast Asian waters.
Five passengers who checked in for Flight MH370 didn't board the plane, and their luggage was removed from it, Malaysian authorities said. Malaysian Transport Minister Hishammuddin Hussein said this also was being investigated, but he didn't say whether this was suspicious.
The search effort, involving at least 34 aircraft and 40 ships from several countries, was being widened to a 100-nautical mile (115-mile, 185-kilometer) radius from the point the plane vanished from radar screens between Malaysia and Vietnam early Saturday with no distress signal.
Two of the passengers were traveling on passports stolen in Thailand and had onward tickets to Europe, but it's not known whether the two men had anything to do with the plane's disappearance. Criminals and illegal migrants regularly travel on fake or stolen documents.
Hishammuddin said biometric information and CCTV footage of the men has been shared with Chinese and U.S. intelligence agencies, which were helping with the investigation. Almost two-thirds of the passengers on the flight were from China.
The stolen passports, one belonging to Christian Kozel of Austria and the other to Luigi Maraldi of Italy, were entered into Interpol's database after they were taken in Thailand in 2012 and 2013, the police organization said.
Electronic booking records show that one-way tickets with those names were issued Thursday from a travel agency in the beach resort of Pattaya in eastern Thailand. Thai police Col. Supachai Phuykaeokam said those reservations were placed with the agency by a second travel agency in Pattaya, Grand Horizon.
Thai police and Interpol officers questioned the owners. Officials at Grand Horizon refused to talk to The Associated Press.
Police Lt. Col. Ratchthapong Tia-sood said the travel agency was contacted by an Iranian man known only as "Mr. Ali" to book the tickets for the two men.
"We have to look further into this Mr. Ali's identity because it's almost a tradition to use an alias when doing business around here," he said.
The travel agency's owner, Benjaporn Krutnait, told The Financial Times she believed Mr. Ali was not connected to terrorism because he had asked for cheapest tickets to Europe and did not specify the Kuala Lumpur to Beijing flight.
Malaysia's police chief was quoted by local media as saying that one of the two men had been identified - something that could speed up the investigation.
Civil aviation chief Azharuddin Abdul Rahman declined to confirm this, but said they were of "non-Asian" appearance, adding that authorities were looking at the possibility the men were connected to a stolen passport syndicate.
Asked by a reporter what they looked like, he said: "Do you know of a footballer by the name of (Mario) Balotelli? He is an Italian. Do you know how he looks like?" A reporter then asked, "Is he black?" and the aviation chief replied, "Yes."
Possible causes of the apparent crash include an explosion, catastrophic engine failure, terrorist attack, extreme turbulence, pilot error or even suicide, according to experts, many of whom cautioned against speculation because so little is known.
Malaysia's air force chief, Rodzali Daud, has said radar indicated that before it disappeared, the plane may have turned back, but there were no further details on which direction it went or how far it veered off course.
On Sunday, a Vietnamese plane spotted a rectangular object that was thought to be one of the plane's doors, but ships could not locate it. On Monday, a Singaporean search plane spotted a yellow object 140 kilometers (87 miles) southwest of Tho Chu island, but it turned out to be sea trash.
Malaysian maritime officials found oil slicks in the South China Sea, but lab tests found that samples of it were not from an aircraft, Azharuddin said.
Selamat Omar, a Malaysian whose 29-year-old son Mohamad Khairul Amri Selamat was a passenger on the flight, told of getting a call from the airline saying the plane was missing.
"We accept God's will," Selamat said. "Whether he is found alive or dead, we surrender to Allah."
Gomez reported from Kuala Lumpur, Malaysia. Associated Press writers Gillian Wong and Louise Watt in Beijing, Joan Lowy in Washington and Scott Mayerowitz in New York contributed to this report.