Malaysian investigators said on Monday that they have still not found anything that could be parts of missing Malaysia Airlines flight MH370, as the search effort entered its third day.
Azharuddin Abdul Rahman, the director-general of Malaysia's Department of Civil Aviation, said hijacking had not been ruled out and all possibilities were being explored in the disappearance of the plane, which he described as an "unprecedented aviation mystery."
"We remain puzzled," he told a news conference in the Malaysian capital Kuala Lumpur. "To confirm what happened to this ill-fated aircraft, we need to see parts of the aircraft. We have not secured any parts of the aircraft today."
The Malaysian authorities gave another update at 8 p.m. local time. They confirmed that oil slicks spotted off the cost of South Vietnam were unrelated to the missing plane.
They also said that from Tuesday, the search area radius for the missing plane would be expanded to 100 nautical miles.
Malaysia's civil aviation chief said: "This is still a search-and-rescue operation" — rather than a disaster-recovery operation, suggesting hopes of recovering passengers alive.
The next press briefing on the missing aircraft is scheduled for 10 a.m. in Kuala Lumpur.
Flight MH370 disappeared early on Saturday, about an hour into its flight from Kuala Lumpur to Beijing, after climbing to a cruising altitude of 35,000 feet.
About 40 ships and roughly 34 aircraft from countries including the U.S., China, Australia and Singapore are taking part in the search effort for the plane carrying 227 passengers and 12 crew members.
Malaysia airlines has said people of 14 nationalities were among the passengers, including at least 152 Chinese, 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
International police agency Interpol confirmed on Sunday that two passengers had boarded the flight using stolen Austrian and Italian passports, raising fears about a possible terrorist attack.
Separately, Malaysian authorities said on Monday that five passengers who had bought tickets failed to board the flight.
Authorities also warned that a stolen-passport syndicate could have been involved. Their identities have not yet been confirmed, but the authorities said they weren't "Asian-looking".
"The two passengers traveling on false passports is a red flag…That's undoubtedly where the focus of the investigation is going to turn as soon as we know more about the location of the crash site," said Richard Aboulafia, vice president of analysis at Teal Group Corporation.
"I think fundamentally, they are searching in the wrong location," Scott Hamilton, founder of aviation consultancy Leeham, told CNBC.
"If the plane has gone down anywhere along the intended flight path, I feel very confident we would have seen debris. The fact that you haven't seen debris suggests to me the airplane crashed some place else, whether on land or some place else in the water," he said.
At the Monday press conference, the authorities said that debris had been noticed by an aircraft south of Hong Kong. Ships are being dispatched to investigate, but the results will not be known until Tuesday.
Fuad Sharuji, vice president of operations control at Malaysia Airlines, said the airline had received several pieces of information that might be related to the missing aircraft, but had so far proved inconclusive.
Vietnam fueled speculation for a breakthrough when it announced it had sent helicopters to investigate a floating yellow object, thought to be a life raft. But the country's civil aviation authority later said the object had proved to be a "moss-covered cap of cable reel."
Reuters meanwhile reported a senior source as saying that officials investigating the disappearance suspected the jet might have disintegrated in mid-flight.
(Read more: Five things to know about the jet)
No distress signal was sent from the lost plane, which experts said suggested a sudden failure or explosion. The jet last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, about an hour after take-off.
Malaysia's air force said that radar-tracking showed the plane might have turned back from its route before it went missing.
"Certainly something unexpected happened and it had to be catastrophic in nature in order to prevent the crew from communicating it," said John Goglia, former member of the National Transportation Safety Board in the U.S.
"That doesn't rule out mechanical failure and it doesn't rule out some sort of activity by person or persons unknown."
Air France 447 parallels
Analysts drew parallels to Air France flight 447, which crashed in the Atlantic midway through a flight in 2009, without sending a distress signal.
"There are many similarities to Air France 447," said Mary Schiavo, an aviation attorney at Motely Rice, and a former inspector general of the U.S. Department of Transportation.
"Except that there's one glaring dissimilarity and that is that we've got no information from the plane and working on Air France 447, we were aided by the fact that the airplane itself sent information back to base," she added.
Peter Goelz, formerly the managing director of the U.S. National Transportation Safety Board, said that parallels could "absolutely" be drawn between the two disasters.
"The ocean is vast: it is very difficult to find something as small as plane… It is going to take weeks, maybe months," he told CNBC.
(Read more: Malaysia Airlines won't ground B777 fleet, CEO says)
Distraught families of loved ones on the flight continued to await news.
"The last time I heard from my son, he called before the flight to say he would be home before 9 a.m.," Lu Zhanzhong, the father of one passenger told CNBC in Beijing.
Another upset relative said: "My grandparents were on the plane and they were nearly 80 years old, we don't have much hope."
Families voiced their frustration about the lack of information at the Beijing crisis center, where friends and relatives awaited news of loved ones. There was also some anger towards Chinese authorities, particularly regarding the absence of senior officials on the ground.
In Kuala Lumpur, shares of Malaysian Airlines dived almost 18 percent to hit a record-low, as markets reacted to the news of the missing aircraft — a Boeing 777.
"This is one of the safest planes ever built in the history of the business and of course the weather in the area was quite clear, so I think we can probably rule out some kind of technical failure of the equipment," said Aboulafia.
"I think we are looking at some kind of human event, unfortunately."
(Slideshow: Latest on missing Malaysian jet in pictures)
Boeing has said it is monitoring the situation and has sent officials to help in the investigation.
- Reuters contributed to this report