The United States sent the FBI to investigate after Malaysia Airlines flight MH370 vanished from radar early Saturday somewhere at sea between Malaysia and Vietnam, but stressed there was no evidence of terrorism yet.
Malaysian authorities also expanded their search for wreckage to the country's west coast, and asked for help from Indonesia. Searches so far had concentrated on waters to the country's east, in the South China Sea.
A total of 40 ships and 22 aircraft from an array of countries including China and the US are now involved in the hunt across the two areas, officials said.
After it emerged that two people boarded the missing flight with stolen European passports, Malaysia's transport minister Hishammuddin Hussein said he was looking at four suspect passengers in all.
He declined to offer details, saying authorities were examining "the entire manifest", but confirmed that the Federal Bureau of Investigation (FBI) was dispatching personnel to Malaysia.
"At the same time our own intelligence has been activated, and of course, the counter-terrorism units... from all the relevant countries have been informed," Hishammuddin said, refusing also to rule out the possibility that the plane may have been hijacked.
The flight went missing about an hour after taking off from Kuala Lumpur, bound for Beijing. A total of 153 Chinese nationals were on board, and relatives camped out at the main international airport in China's capital bemoaned the lack of news Sunday.
The flight was carrying 227 passengers and 12 crew. A US company based in Texas said 20 of its employees were among the missing passengers -- a dozen from Malaysia and eight from China.
Thirty-eight Malaysians and seven Indonesians were aboard, as well as a range of other nationalities including Australian, Indian, American, Dutch, and French.
If the worst is confirmed, it would be the second fatal crash in the nearly 20-year history of the popular Boeing plane. A 777-200 operated by South Korea's Asiana Airlines skidded off the runway after hitting a sea wall in San Francisco last year, killing three people.
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