Two mystery passengers bought their tickets together, and boarded the missing Malaysia Airlines flight MH370 apparently using stolen passports. The effort to answer the pressing questions of who they are and what happened to the aircraft over the South China Sea on Saturday morning was focusing on the examination of CCTV footage showing the two travellers on Sunday night as officials also probed the identity of two other unknown passengers.

More than a day and half after the Boeing 777 went missing an hour into its flight between Kuala Lumpur and Beijing, officials in Malaysia said they had been unable to find any confirmed debris or other trace of the jet.

They said an investigation as to whether an oil slick spotted on the surface of the South China Sea was from the aircraft was ongoing. Meanwhile, the authorities in Vietnam said a military plane had spotted an object suspected to be part of the missing airliner off the Ca Mau peninsula. Efforts to reach the debris will continue Monday.

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Amid speculation that foul play may have been behind the plane’s demise, Director-General of Malaysia’s civil aviation body, Azharuddin Abdul Rahman, said Sunday night that Malaysian investigators were looking at CCTV footage of two passengers who boarded the plane with stolen passports.

“The video of the two passengers is now being looked at,” he said in Kuala Lumpur. “We are looking at all angles and all possibilities.”

Mr Rahman declined to provide further details of what examination of the CCTV footage had so far shown. But he added: “On the possibility of hijack, we are not ruling any possibility. However, it is important to state that our main concern is to focus our effort to ﬁnd the missing aircraft.”

In the hours after flight MH370 went missing, it emerged that two Europeans listed on the passenger manifest – an Italian, Luigi Maraldi and an Austrian, Christian Kozel – had not been on the flight.

Mr Maraldi’s passport had been stolen in Thailand last year and Mr Kozel’s was stolen two years ago. Interpol said both documents were listed in its stolen passport database.

The two passports were reportedly used to buy contiguous tickets from China Southern Airlines – which codeshares with Malaysia Airlines – from a travel agent in Pattaya in Thailand.

The itinerary included an onward flight from Beijing to Amsterdam. The ticket bought using the Italian passport then goes on to Copenhagen while the ticket bought with the Austrian passport continues to Frankfurt.

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While China has a reputation for being rigorous on visa approvals, recently introduced rules allow citizens of many Western nations visa-free entry for 72 hours upon arrival in Beijing as long as they have an onward ticket.

The Reuters news agency said Malaysian investigators, assisted by the FBI, are focusing efforts on the identities of four passengers in particular. The four consist of the two travellers using the stolen Austrian and Italian passports, and two passengers also using European passports, possibly Ukrainian.

The international investigative agency Interpol said that the Austrian and Italian passport had been added to its database of stolen documents in 2012 and 2013. But it said no checks were made by either country between the time the passports were entered as stolen and them being used to board MH 370. As a result, it could not say how many times the documents may have been previously used.

It said investigators were currently examining whether any of the other passports used to board the Malaysia Airlines flight were stolen.

The 11-year-old plane, which was carrying 239 people, lost contact with ground controllers somewhere between Malaysia and Vietnam after leaving Kuala Lumpur at around 12.40am on Saturday morning. The list of passengers include 154 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians, four Americans and two Canadians. There were also 12 crew members.

Among the Chinese travellers was a 19-member group of artists returning home after an exhibition.

The search area for the missing plane has been extended to 50 nautical miles of the last point of contact of the plane, 120 miles from the Malaysian town of Kota Bharu. A total of 40 ships and 34 aircraft from countries including Thailand, Indonesia, China and the US are involved in the search.

There has been speculation that something caused the plane to explode and disintegrate while at cruising altitude – something that would account for the absence of debris and the fact that the pilots did not send a distress signal. Meanwhile, the Malaysian military said radar images suggested that the plane had started to turn back.

Mr Rahman said the main priority of investigators was to locate any wreckage from the plane. Officials would then be able to examine the “black-box” flight recorder to ascertain further clues. Crucially, no signal has been received from the plane’s emergency locator transmitter.

Andrew Charlton, an aviation consultant at Geneva-based Aviation Advocacy, said there was much that did not add up.

“The 777 is a very reliable aircraft, Malaysia is a very good airline and it had cleared take-off and landing. For the aircraft not to have been able to talk to the ground is really most alarming and concerning,” he said. “It just disappeared off the face of the map. When this happens it’s catastrophic and instantaneous, and it’s very difficult not to assume an explosion was involved at that point.”

If Airlines flight died, then it will be the deadliest aircraft accident since November 12, 2001, when American Airlines Flight 587 crashed in New York, killing all 260 people on board and five on the ground.