Malaysian officials are poring over CCTV footage and questioning immigration officers and guards at Kuala Lumpur's international airport, concerned that a security breach may be connected to the disappearance of Malaysia Airlines flight MH370.
At least two passengers on the Beijing-bound jet have been found to be using stolen passports, and Malaysia's transport minister, Hishamuddin Hussein, confirmed on Sunday that investigators were looking at four passengers in total.
The US sent the FBI to investigate after the plane en route from Kuala Lumpur to Beijing vanished from radar early on Saturday somewhere at sea between Malaysia and Vietnam, but stressed there was no evidence of foul play yet.
The four passengers investigated by authorities comprise two travellers with European passports, possibly Ukrainian, in addition to two travelling on stolen Austrian and Italian passports, two Malaysian officials with knowledge of the investigation told Reuters news agency.
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"We have deployed our investigators to look through all the security camera footage. Also, they are interviewing immigration officials who let the imposters through," said one official with direct knowledge of the investigation.
"Early indications show some sort of a security lapse, but I cannot say any further right now."
The head of Malaysia's civil aviation authority told reporters on Sunday that two "imposters" had been identified by investigators as they made their way from check-in, through immigration to the departure gate.
Interpol said in a statement that the international police body was investigating all passports used to board flight MH 370 and was working to determine the "true identities" of the passengers who used the stolen passports.
Possible wreckage found
Searchers in a low-flying plane spotted an object that appeared to be one of the plane's doors, the state-run Thanh Nien newspaper said, citing the deputy chief of staff of Vietnam's army, Lieutenant General Vo Van Tuan.
Vietnamese searchers on ships worked throughout the night but could not find the object, spotted on Sunday afternoon.
|Search continues for sign of missing Malaysian aircraft
Doan Huu Gia, the chief of Vietnam's search and rescue coordination centre, said on Monday that four planes and seven ships from Vietnam were searching for the object but nothing had been found.
Two ships from the maritime police were headed to the site about 90km south of Tho Chu island in the Gulf of Thailand, the same area where oil slicks were spotted Saturday.
Al Jazeera's Scott Heidler, reporting from Kuala Lumpur said the authorities would not make any firm announcements until they had hard evidence.
"Some of the items found in the water turned out to be nothing. Most recently though, the Vietnamese saw something they believed to be part of a door. That has not been verified yet," he said.
"The oil slicks have not been verified as being part of the craft. It is being analysed right now as to if it could be from an aircraft."
Malaysia earlier in the day said the plane may have inexplicably turned back.
"There is a distinct possibility the airplane did a turn-back, deviating from the course," said Malaysia's air force chief, General Rodzali Daud, citing radar data.
But Malaysia Airlines chief executive Ahmad Jauhari Yahya said the Boeing 777's systems would have set off alarm bells.
"When there is an air turn-back the pilot would be unable to proceed as planned," the AFP news agency quoted him as saying, adding that authorities were "quite puzzled" over the situation.
Malaysian authorities have expanded their search for wreckage to the country's west coast after initially concentrating to the east in the South China Sea.
A total of 40 ships and 22 aircraft from an array of countries including Malaysia's neighbours, China and the US are now involved in the hunt across the two areas, officials said, with two Australian surveillance aircraft also due to join the search.