Media playback is unsupported on your device Media caption The BBC's John Sudworth: "Family members have been told to prepare for the worst"

Radar signals show that a Malaysia Airlines plane missing for more than 24 hours may have turned back, Malaysian officials have said.

Rescue teams looking for the plane have now widened their search area.

Investigators are also checking CCTV footage of two passengers who are believed to have boarded the plane using stolen passports.

Flight MH370 from Kuala Lumpur to Beijing disappeared south of Vietnam with 239 people on board.

Analysis Malaysia Airlines lost contact with flight MH370 for five hours before it confirmed the news. After that, the authorities began to hold regular news briefings but they were cautious about what they revealed. The slow pace of information forced Malaysians to turn to social media first - then ask the authorities to confirm speculation or reports that appeared online. Among the many questions was how two passengers with fake European passports could have boarded flight MH370. Over the past four years, I have travelled frequently through the same airport. As a Canadian passport holder I have to scan both index fingers before I enter the country but not when I leave. The biometric system was set up in 2011 to prevent foreigners from repeatedly coming in to work illegally and to curb human trafficking and wildlife smuggling. Malaysian officials say they are working hard to answer questions. They have reminded people to avoid speculation, but it hasn't reassured distressed family members. What we know about Flight MH370

Air and sea rescue teams have been searching an area of the South China Sea south of Vietnam for more than 30 hours, but there have been no definite sightings of wreckage.

Late on Sunday, the Vietnamese authorities said a navy aircraft had spotted "an object" suspected of belonging to the missing plane, but officials said it was too dark to be certain.

The object is thought to be near a potential oil slick that was spotted on Saturday, but again officials have urged that this may be nothing to do with flight MH370.

Malaysia's civil aviation chief, Azharuddin Abdul Rahman, told a press conference in Kuala Lumpur that the search area had been expanded to include the west coast of Malaysia.

Five passengers booked on the flight did not board, he added. Their luggage was consequently removed.

There are now 40 ships and 34 aircraft from nine different nationalities taking part in the search. But no signal has been received from the plane's emergency locator transmitter, Malaysian aviation authorities say.

Air force chief Rodzali Daud said the investigation was now focusing on a recording of radar signals that showed there was a "possibility" the aircraft had turned back from its flight path.

Fake passports

The BBC has confirmed that a man falsely using an Italian passport and a man falsely using an Austrian passport purchased tickets at the same time, and were both booked on the same onward flight from Beijing to Europe on Saturday.

Media playback is unsupported on your device Media caption The BBC's Alice Budisatrijo describes the growing search effort

Media playback is unsupported on your device Media caption "There is a possibility the aircraft did make a turn back"

Both had purchased their tickets from China Southern Airlines, which shared the flight with Malaysia Airlines, and they had consecutive ticket numbers.

The real owners had their passports stolen in Thailand in recent years.

Image copyright Getty Images Image caption Aerial search teams may have spotted wreckage but ships must wait until dawn to confirm

Image copyright Reuters Image caption Military ships have joined the search off Vietnam

Image copyright Reuters Image caption Developments have been the subject of intense media attention in Beijing

The international police agency Interpol confirmed that at least two passports recorded as lost or stolen in its database were used by passengers on the flight - and that no checks of its database had been made for either passport between the time they were stolen and the departure of the flight.

Manifest for Flight MH370 153 Chinese including one child

38 Malaysians

7 Indonesians

6 Australians

5 Indians

4 French

3 Americans including one child

2 each from New Zealand, Ukraine and Canada

One each from Russia, Taiwan, Italy, Netherlands and Austria ( although both Italy and Austria deny any of their nationals were onboard) Source: Malaysia Airlines Who were passengers?

"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in Interpol databases," the agency's Secretary General Ronald Noble said in a statement.

He expressed frustration that few of Interpol's 190 member countries "systematically" search the database.

Malaysia's Transport Minister Hishammuddin Hussein said international agencies including the FBI had joined the investigation and all angles were being examined.

"Our own intelligence have been activated and, of course, the counterterrorism units... from all the relevant countries have been informed," he said.

"The main thing here for me and for the families concerned is that we find the aircraft."

The passengers on the flight were of 14 different nationalities. Two-thirds were from China, while others were from elsewhere in Asia, North America and Europe.

Boeing 777 fact sheet Twin-engine jet launched in June 1995

One of the world's most popular long-distance planes

Seats between 300 and 380 passengers

Has flown around five million flights

Often used for non-stop flights of 16 hours or more

In September 2001 a crew member died in a re-fuelling fire on a 777 at Denver International Airport

In 2013 three Chinese women died when the 777 Asiana Flight 214 crashed in San Francisco

The plane vanished from radar south of Vietnam at 17:30 GMT Friday (01:30 local time Saturday).

Malaysia Airlines had previously said it last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu.

Distraught relatives and loved ones of those on board are being given assistance at both the arrival and departure airports.

Many have expressed anger at the lack of information.

"I can't understand the airline company. They should have contacted the families first thing," a middle-aged woman told AFP news agency at Beijing airport, after finding out her brother-in-law was on the flight.

Texas firm Freescale Semiconductor says 20 of its Malaysian and Chinese employees were on the flight, according to a statement on its website.

Malaysia's national carrier is one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.

Correspondents say the route between Kuala Lumpur and Beijing has become more and more popular as Malaysia and China increase trade.