Suspected fragments from the missing Malaysia Airlines plane carrying 239 people have been found off Vietnam.
The Vietnamese navy said objects, one thought to be an aircraft door, were spotted by a rescue plane off the country's south coast.
Security services are investigating whether the Boeing 777-200 was destroyed in a terror attack.
Interpol said at least two passports used on the flight were stolen and it is "examining additional suspect passports".
The international police agency said it was of "great concern" that passengers were able to board the plane using stolen passports, and no checks were made against its database.
Flight MH370 disappeared two days ago off Vietnam's south coast.
The search area has been widened after radar data indicated the plane may have turned back.
Officials investigating the plane's disappearance suspect it may have disintegrated at 35,000ft, according to Reuters.
A source involve in the probe said: "The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet."
The source said that if the aircraft had plunged into the sea and broken up on impact, search teams would have expected to find a concentrated pattern of debris.
The FBI and Boeing have joined the investigation after it was revealed four passengers may have been travelling on false passports.
Malaysia's defence and transport minister Hishamuddin Hussein told a news conference in Kuala Lumpur: "All the four names are with me."
Asked whether he believes the plane was hijacked, he would only say: "We are looking at all possibilities."
It emerged on Saturday that two men boarded the plane using stolen European passports.
They bought their tickets together and paid for them in Thai baht, Sky News has learned, and were due to fly on to Europe from Beijing.
The plane was travelling from Kuala Lumpur to Beijing when it vanished around two hours into the flight.
The plane disappeared from radar at 1.30am (5.30pm UK time) on Friday, about 85 miles (135km) north of the Malaysian city of Kuala Terengganu.
A huge search involving 22 aircraft and 40 ships is continuing in the vast waters of the Gulf of Thailand, between Vietnam and Malaysia.
It concentrated around the Vietnamese island resort of Phu Quoc after Vietnamese air force jets spotted two huge oil slicks.
The parallel slicks - both between 10 miles (16km) and 12 miles (19km) long and 500 metres apart - were consistent with the kind of spills caused by fuel from a crashed airliner, a Vietnamese government statement said.
The search has now widened to the sea off Malacca, on the west coast of Malaysia, after radar data indicated the plane may have turned back before disappearing.
US federal safety officials said a team of experts are heading to Asia to help in the investigation.
The team includes accident investigators from the National Transportation Safety Board, as well as technical experts from the Federal Aviation Administration and Boeing.
Earlier today, Malaysia's Civil Aviation chief Azaharudin Abdul Rahman said search teams have not found any debris from the plane.
He said no other aircraft in the Malaysia Airlines fleet would be grounded and indicated there were "no abnormalities" in the data received from the flight.
Two-thirds of the jet's passengers were from China. The rest were from elsewhere in Asia, North America and Europe.
The plane's disappearance is especially mysterious because it happened when the plane was at cruising altitude, not during the more dangerous phases of take-off or landing.
Officials are examining CCTV footage of passengers boarding the plane.
One of the passengers was listed as a 37-year-old Italian called Luigi Maraldi but he has contacted his parents to say he was not on the airliner.
He had his passport stolen in Thailand several months ago, leaving questions over who used his passport to board the plane and whether that has anything to do with the airliner's disappearance.
Another passenger used a passport belonging to Austrian citizen Christian Kozel, whose passport was stolen in Thailand two years ago.
He is listed as one of the passengers although he has been confirmed as safe and well by authorities.
Relatives are still waiting anxiously at Beijing airport for news of their loved ones.
Tech firm Freescale Semiconductor said 20 of its staff were on the plane.
In a statement it said: "Twelve are from Malaysia and eight are from China. The entire Freescale Semiconductor community is deeply saddened by this news."
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