Calum MacLeod

USA TODAY

KUALA LUMPUR, Malaysia — Chinese aviation authorities arrived here Monday to assist in an investigation of a missing jetliner that it has complained is going slow as investigators zeroed in on two mystery passengers traveling aboard the Malaysia Airlines flight.

Authorities questioned travel agents Monday at a beach resort in Thailand about two men who boarded the doomed plane with stolen passports and one-way tickets bound for Europe.

Electronic booking records show the tickets were issued Thursday from a travel agency in the beach resort of Pattaya in eastern Thailand at the request of a second travel agency in Pattaya, Grand Horizon, Thai police Col. Supachai Phuykaeokam said.

The Financial Times, citing Thai police, said the ticket purchase was made over the phone by an Iranian man named Kazem Ali, who used the stolen passports he said were those of two friends who wanted to return home to Europe. The tickets were paid for in cash, the Times said.

Global Horizon's owner, Benjaporn Krutnait, said she believed Ali was not connected to terrorism because he had asked for the cheapest tickets to Europe and did not specify the Kuala Lumpur to Beijing flight.

Malaysia's police chief told news media in Kuala Lumpur that one of the two men had been identified. Civil aviation chief Azharuddin Abdul Rahman said video of the men believed to be traveling with the stolen passports showed that they were of "non-Asian" appearance.

Interpol, the international police agency, confirmed that the stolen passports were entered into its database in 2012 and 2013 but that no authorities in Malaysia -- or any other country -- had checked.

Rahman stressed that it is not known whether the two men had anything to do with the plane's disappearance. Illegal migrants and criminals often use forged or stolen passports.

The FBI will run the fingerprints and photos through its databases in hopes of identifying the men.

A friend who hosted the two Iranians in Kuala Lumpur after they arrived from Tehran told the BBC Persian service that both men had bought the stolen passports in the Malaysian capital and wanted to settle in Europe, The Telegraph in London reported. One wanted to travel to Frankfurt, Germany, where his mother lived, while the other was headed to Denmark, both by way of Amsterdam.

The Boeing 777 with 239 people on board disappeared Saturday over the South China Sea less than an hour after it took off from Kuala Lumpur on its way to Beijing. No debris had been found as of late Monday. A search effort involves dozens of aircraft and ships from several countries.

The search area for the missing jetliner expanded to the western coast of the country, Malaysian Airlines said in a statement Tuesday. That brings the focus of the hunt beyond the flight path, and to the opposite side of the country from where the flight was reported missing.

Civil aviation chief Azharuddin Abdul Rahman says search teams continued to look in both areas.

On Monday, Chinese diplomat Guo Shaochun arrived with a working group from the Chinese ministries of foreign affairs, transport, public security and the civil aviation administration. Most of the passengers aboard the flight with Chinese.

Guo said he hoped his team would help improve Malaysia's investigation. China foreign ministry spokesman Qin Gang urged Malaysia "to step up their efforts to speed up the investigation and provide accurate information to China in a timely fashion."

The Global Times, a Chinese Communist Party newspaper, assailed an investigation it alleged, "was not swift enough." Chinese relatives of missing passengers arrived here early Tuesday in the hopes of finding out more.

"The airport and the whole country is sad," said Lin Mohammed, a tourism representative at Kuala Lumpur airport. "We have several races, Indian, Chinese Malay, and different religions, but they united in response," she said.

"Everybody is hoping to find the plane, and to find it safe."

Possible causes of the apparent crash include an explosion, engine failure, terrorist attack, violent turbulence, pilot error or pilot suicide. Malaysia's air force chief, Rodzali Daud, has said that radar indicated the plane may have turned back just before it vanished but there were no further details.

Online speculation in China has suggested without evidence that the culprits were Uighurs, an ethnic group in northwest China of which some members are blamed for terror attacks.

Although 10 countries are involved in the search, Malaysia will likely lead the investigation unless it defers to another country, say crash investigators and lawyers. According to the International Civil Aviation Organization, a part of the United Nations, the location of the mishap dictates who leads the investigation. This is why the U.S. National Transportation Safety Board is leading for the Asiana Airlines crash in San Francisco in July.

If the crash happens in international waters such as the Gulf of Thailand, where the search is focused, international law dictates that the plane's country of registry — Malaysia — leads the investigation.

Contributing: Thomas Maresca in Vietnam; Bart Jansen in Washington; the Associated Press