As the search for a missing Malaysian airliner entered its third day, the head of Malaysia's Civil Aviation Authority called the disappearance of flight MH370 an "unprecedented mystery".
The plane disappeared from radar Saturday morning about an hour after it took off from Kuala Lumpur en route to Beijing.
Azharuddin Abdul Rahman said Monday that the massive air and sea search has failed to find any trace of the plane or 239 people on board.
Interpol confirmed at least two passengers on the flight used stolen passports and authorities are checking to see whether others aboard used false identity documents.
Reuters quoted the chief Malaysian investigator as saying the two men who used stolen passports to board the jetliner were not of Asian appearance. Airport CCTV footage showed they completed all security procedures, he said.
Azharuddin said a hijacking attempt could not be ruled out.
A senior source involved in preliminary investigations in Malaysia said the failure to find any debris indicated the plane may have broken up mid-flight, which could disperse wreckage over a very wide area, Reuters reported.
The news agency also quoted a U.S. government source as saying that the United States has reviewed imagery taken by American spy satellites for evidence of a mid-air explosion, but saw none.
China urged Malaysia to step up the search for the missing plane and has sent security agents to help with the investigation into the misuse of passports. More than 150 Chinese nationals were on the flight. China has sent four search-and-rescue vessels and two warships to help in the mission.
In all, eight countries joined the search for the plane early Saturday, but so far no positive sightings of the jetliner have been made. Malaysia’s Department of Civil Aviation said the eight nations have a combined 40 ships and 34 aircraft involved in the hunt.
Several sightings of suspicious objects were reported Saturday and Sunday, including what was believed to be a window or door of the plane. But Azharuddin said nothing has been verified.
"We have not found anything that appears to be objects from the aircraft, let alone the aircraft. We will be intensifying our efforts to locate the missing aircraft," he told reporters.
He said some samples of an oil slick spotted in the area are being analyzed to see if they could have come from the aircraft.
"The honorable prime minister used the word perplexing," said Azharuddin. "We are equally puzzled as well and to be confirmed what really happened on that particular day, on this ill-fated aircraft, we need hard evidence, we need concrete evidence, we need parts of the aircraft."
Vietnam dispatched two planes and seven ships to search for the plane, the Ministry of Foreign Affairs said in a statement Monday. The National Committee for Search and Rescue said another five planes and four ships are on standby for search activities.
“The wreckage is very unlikely to show up on radar, and it is also very unlikely to show in infra red, because it has the same temperature as the surface," said Greg Waldron, Asia managing editor of Flightglobal, a trade publication for the aviation sector. "So in terms of finding pieces of the aircraft, if indeed these pieces of aircraft are floating around in the sea, you are really relying on people's eyeballs. And also the wreckage if there is wreckage has had days to spread. And this could make it more challenging to locate."
The United States sent the USS Pinckney, an Arleigh Burke-class guided-missile destroyer, to the area on Sunday. Another vessel is on its way, according to Bleu Moore, spokesman from the 7th Fleet public affairs office.
"We’re also sending out the USNS Ericsson. It’s on its way there, it’s not in the same body of water but its on the way there," said Moore.
He said the biggest challenge is time: the more time passes, the less chance there is to save lives.
It could be a long wait before answers are found. When an Air France jetliner disappeared over the Atlantic on its way from Rio de Janeiro to Paris in 2009, it took investigators nearly two years to find and retrieve the aircraft’s black box data recorders.
(Reuters contributed to this report.)