It has been more than a day and a half since Malaysia Airlines Flight 370 vanished on its way to Beijing, and the fate of the more than 200 people on board is still unknown. A massive search and rescue is under way in waters off the coast of Malaysia and Vietnam where the plane presumably crashed. As the hours pass there are more questions than answers about what happened.
Malaysian authorities say an expansive and intensive search for the plane is under way with support from four other countries, including China and the United States. Authorities say some 40 ships and 22 planes are helping out.
Much of the focus so far has been on waters off Vietnam's southwestern coast, where a large oil slick has been spotted, but now Malaysian authorities say the search effort has extended to land and sea off the western Malaysian coast near Penang.
"What we have done is actually look into the recording on the radar that we have and we realized there is a possibility the aircraft did make a turnback," said Royal Malaysian Air Force chief Rodzali Daud.
Aviation safety experts say the possibility that the plane turned back at cruising altitude suggests that something had gone wrong and that it may have been trying to make an emergency landing.
At a news conference Sunday afternoon, Malaysia Airlines told families to expect the worst while stressing that search and rescue efforts continue. The airline is making arrangements to take family members to Malaysia by Monday morning at the earliest.
Malaysia Airlines says a disaster recovery specialist from the United States will be assisting the carrier. It is also setting up command centers in Malaysia's Kota Bharu and Vietnam's Ho Chi Minh city.
The revelation that two of the passengers on board were using stolen passports, and questions about the identity of two others has even raised concerns that the flight's disappearance was the result of a terrorist act.
Authorities in Italy and Austria have confirmed that two passengers who were thought to have been on the flight were not - but both had their passports stolen in the past two years.
Malaysian Transport Minister Hishammuddin Hussein, who is also the country's defense minister says he has spoken with international intelligence agencies about the four names.
"I am in touch with the international intelligence agencies and at the same time our own intelligence has been activated and of course the counter terrorism units CTIs and CTUs from all the relevant countries will be, have been informed, That's what I've been doing since yesterday," he said.
Hussein says terrorism is not the sole focus of the investigation at this point and adds that authorities are not ruling out any possibilities. Authorities say they are reviewing security footage in the airport of the two individuals who boarded the plane using stolen passports.
Analysts say that while Asian airport security is not as tight as it is in Europe or the United States, the possibility that as many as four passengers got on board using stolen documents will raise some difficult questions.
No group or organization has come forward to claim responsibility for the plane's disappearance. Authorities in China will be watching the investigation closely.
The plane was headed to Beijing at a time of already heightened security in the capital. Last week, China's leaders began key annual political meetings in Beijing.
Airline safety experts say it is unusual for a plane to crash after reaching cruising altitude. Daniel Tsang is the founder and chief analyst at Aspire Aviation.
"Most incidents or accidents take place during take off or landing, when it is the most dangerous part of the entire duration of the flight," said Tsang. "Because during take off and landing there are so many factors, and you can have runway overruns, etc."
Tsang says that searches at sea for lost planes are tremendously difficult. Once the wreckage is found, however, the way the debris is dispersed will give clues as to what might have happened during the flight.
"If the airplane hits the ocean in one piece, it remains intact, most likely the debris area would be very small and concentrated in a small diameter or circle," said Tsang. " But if you are talking about an inflight break up as some are speculating on the MH370 you'd be looking for miles and miles of diameter of debris field."
The Malaysia Airlines flight from Kuala Lumpur to Beijing was a Boeing 777-200, a plane air safety experts say has an almost spotless record. There were also no signs of bad weather during the flight. Authorities in Malaysia say they lost contact with the plane about one hour after its departure from Kuala Lumpur.