New information from the Thai government bolsters the belief that missing Malaysia Airlines Flight 370 took a sharp westward turn after communication was lost.
The Thai military was receiving normal flight path and communication data from the Boeing 777-200 on its planned March 8 route from Kuala Lumpur to Beijing until 1:22 a.m., when it disappeared from its radar.
Six minutes later, the Thai military detected an unknown signal, a Royal Thai Air Force spokesman told CNN. This unknown aircraft, possibly Flight 370, was heading the opposite direction.
Malaysia says the evidence gathered so far suggests the plane was deliberately flown off course, turning west and traveling back over the Malay Peninsula and out into the Indian Ocean.
But investigators don't know who was at the controls or why whoever it was took the plane far away from its original destination.
The Thai data is the second radar evidence that the plane did indeed turn around toward the Strait of Malacca.
It follows information from the Malaysian Air Force that its military radar tracked the plane as it passed over the small island of Pulau Perak in the Strait of Malacca.
"The unknown aircraft's signal was sending out intermittently, on and off, and on and off," the spokesman said. The Thai military lost the unknown aircraft's signal because of the limits of its military radar, he said.
The radar data is an encouraging sign that investigators are on the right track, but they still are not sure where the plane ended up.
The latest findings say the plane's last known location detected by a satellite is somewhere along two wide arcs: one stretching north over Asia and the other south into the Indian Ocean. The plane's last electronic connection with the satellite was about six hours after it last showed up on Malaysian military radar.
The total area now being searched stands at 2.24 million square nautical miles, Hishammuddin Hussein, the Malaysian defense and transport minister, said at a news conference Tuesday. That's the same as 2.97 million square miles, or an area nearly the size of the continental United States.
"This is an enormous search area," Hishammuddin said. "And it is something that Malaysia cannot possibly search on its own. I am therefore very pleased that so many countries have come forward to offer assistance and support to the search and rescue operation."
China clears citizens
China says it has found no evidence that any of its citizens on board the missing plane were involved in hijacking or terrorism.
Background checks on all passengers from the Chinese mainland on the plane have found nothing to support such suspicions, Huang Huikang, the Chinese ambassador to Malaysia, said Tuesday, according to the state-run Chinese news agency Xinhua.
Authorities have said they are investigating all 239 people who were on board the flight, which disappeared more than 10 days ago.
According to the airline, 153 of the 227 passengers on board the plane came from mainland China or Hong Kong.
By effectively ruling out suspicions for a large majority of the passengers, Chinese authorities appear to have significantly shortened the list of possible suspects in the investigation.
The Chinese ambassador's statement is also likely to greatly dampen speculation that Uyghur separatists from China's far western region of Xinjiang might have been involved in the plane's disappearance.
One of the two long corridors where authorities say the plane was last detected stretched over Xinjiang, and unconfirmed reports had suggested the possibility that Uyghurs might be connected to the case.
Chinese authorities have accused separatists from Xinjiang of carrying out a terrorist attack this month in which eight attackers armed with long knives stormed a train station in Kunming, a city in southwestern China, killing 29 people and wounding more than 140.
China said Tuesday that it had begun to search for the plane in the parts of its territory that fall under the northern corridor, deploying satellite and radar resources.
Experts are analyzing past and present data along the arc stretching through Chinese territory, Hong Lei, a Foreign Ministry spokesman, said at a news briefing Tuesday in Beijing.
Turn made by computer?
The pilot and first officer of the missing plane, both of them Malaysian, have come under particular scrutiny in the search for clues. Investigators say that whoever flew the plane off course for hours appeared to know what they were doing.
But officials have so far reported no evidence to tie the pilot and first officer to the plane's disappearance.
Supporting the case that whoever took the plane off course had considerable aviation expertise, The New York Times reported that the aircraft's first turn to the west was carried out through a computer system that was most likely programmed by somebody in the cockpit.
An aviation expert, writing an op-ed for CNN.com, floated the idea last week that whoever changed the plane's course was an expert.
The person who programmed the change of course would have been somebody "knowledgeable about airplane systems," The Times reported, citing unidentified American officials.
The information has increased investigators' focus on the pilot and first officer, the newspaper reported. CNN wasn't immediately able to confirm the report.
Asked about the report Tuesday, Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said: "As far as we're concerned, the aircraft was programmed to fly to Beijing. That's the standard procedure."
But he didn't rule out the possibility the flight path had been reprogrammed.
"Once you're in the aircraft, anything is possible," he said.
-- CNN's Kocha Olarn, Steven Jiang, Sarita Harilela and Mitra Mobasherat contributed to this report.
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