Investigators in Malaysia are skeptical the Malaysian airliner that disappeared Saturday was the target of an attack, according to U.S. and European government sources close to the probe, according to Reuters news agency.
Neither the Malaysian agency leading the investigation nor spy agencies in the United States and Europe have ruled out the possibility that the aircraft was intentionally downed.
However, Malaysian authorities have indicated mechanical or piloting problems could be reasons for the apparent crash, the U.S. sources said.
A U.S. source said one reason Malaysian authorities are leaning away from the act of terror theory is because electronic evidence indicates the jetliner may have made a turn back towards Kuala Lumpur before it disappeared.
The search for flight MH370 is in its third day and the chief Malaysian investigator called its disappearance an "unprecedented mystery".
The plane vanished from radar Saturday morning about an hour after it took off from Kuala Lumpur en route to Beijing.
Azharuddin Abdul Rahman said the search area will be significantly expanded Tuesday after days of looking with no sign of the missing plane.
Dozens of ships and planes from several countries have been searching within a 92 kilometer radius from the point where the plane disappeared over the South China Sea.
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.
The Associated Press reports that authorities questioned travel agents Monday at a beach resort in Thailand about the two men, part of a growing international investigation into what they were doing on the flight.
AP also reported that five passengers who checked in for Flight MH370 didn't board the plane, and their luggage was removed from it, Malaysian authorities said. Malaysian Transport Minister Hishammuddin Hussein said this also was being investigated, but he didn't say whether this was suspicious, according to the AP report.
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.
A U.S. government source said 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.
A Malaysian police official displays a photograph of 19-year-old Iranian Pouri Nourmohammadi, one of the two men who boarded missing Malaysia Airlines MH370 flight using stolen European passports.
A Malaysian police woman holds up a picture of Pouri Nourmohammadi, 19, an Iranian who boarded the now missing Malaysia Airlines jet MH370 with a stolen passport.
This combination of images released by Interpol and displayed by Malaysian police in Sepang, Malaysia, on March 11, 2014, shows Pouri Nourmohammadi, 19, (left) and Delavar Seyedmohammaderza, 29, who allegedly boarded the now-missing Malaysia Airlines jet
Military officer Duong Van Lanh works onboard a Vietnamese airforce AN-26 during a mission to find the missing Malaysia Airlines flight MH370 off Tho Chu islands March 11, 2014.
A Chinese relative of passengers aboard a missing Malaysia Airlines plane looks out as she waiting for the latest news inside a hotel room for relatives or friends of passengers aboard the missing airplane in Beijing, China, March 11, 2014.
Family members comfort Chrisman Siregar, left, and his wife Herlina Panjaitan, the parents of Firman Siregar, one of the Indonesian citizens registered on the manifest of the Malaysia Airlines jetliner flight MH370 that went missing, Medan, North Sumatra, Indonesia, March 9, 2014.
Malaysia's Department of Civil Aviation's Director General Azharuddin Abdul Rahman briefs reporters at a press conference on search and recovery efforts within existing and new areas for the missing Malaysia Airlines plane,Sepang, Malaysia, March 10, 2014.
A family member of passengers aboard a missing Malaysia Airlines plane wipes her tears at a hotel in Putrajaya, Malaysia, March 10, 2014.
Chinese relatives of passengers aboard a missing Malaysia Airlines plane wait for the latest news inside a hotel room, Beijing, China, March 10, 2014.
CEO of Malaysia Airlines Ignatius Ong, center, gestures as he prepares to speak to the media near a hotel room for relatives or friends of passengers aboard a missing Malaysia Airlines airplane, Beijing, China, March 10, 2014.
People hold a banner and candles during a candlelight vigil for passengers aboard a missing Malaysia Airlines plane in Kuala Lumpur, Malaysia, March 10, 2014.
Malaysia's Department of Civil Aviation director general Azharuddin Abdul Rahman, second from left, speaks during a press conference at a hotel in Sepang, Malaysia, March 10, 2014.
Italian Luigi Maraldi, left, whose stolen passport was used by a passenger boarding a missing Malaysian airliner, shows his passport as he reports himself to Thai police Lt. Gen. Panya Mamen, right, at Phuket police station in Phuket province, southern Thailand, March 9, 2014.
A U.S. Navy helicopter lands aboard Destroyer USS Pinckney during a crew swap before returning to a search and rescue mission for the missing Malaysian airlines flight MH370 in the Gulf of Thailand, March 9, 2014.
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.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.
Some information in this report was contributed by VOA's Marianne Brown and Reuters.