Radar stations at airports are designed to track planes up to about 60 miles. They are used to help sequence and space landing aircraft. Another series of stations called air route surveillance radar can track planes 200-250 miles away, depending on weather and the age of the technology. Station locations are selected to allow for a slight overlap so planes in heavy-traffic areas are never out of reach of radar.
While radar black spots can exist, experts said the plane's transponders normally would have been emitting signals that would have been picked up by civilian radar. The fact that it apparently wasn't detected suggests they were either disabled or switched off. Planes with no transponders can still be tracked by radar.
Low-flying planes can sometimes avoid radar detection. There is no set height they must be under, but the farther away they are from a radar station, the higher they can be because of the angle of the radar antenna and the curvature of the Earth.
Malaysian police chief Khalid Abu Bakar, who has been ordered to look at possible criminal aspects in the disappearance of Flight MH370, said hijacking, sabotage and issues related to the pilots' psychological health were all being considered.
An Australian TV station reported that the first officer on the missing plane, Fariq Abdul Hamid, had invited two women into the cockpit during a flight two years ago. One of the women, Jonti Roos, described the encounter on Australia's "A Current Affair."
Roos said she and a friend were allowed to stay in the cockpit during the entire one-hour flight on Dec. 14, 2011, from Phuket, Thailand, to Kuala Lumpur. She said the arrangement did not seem unusual to the plane's crew.
"Throughout the entire flight, they were talking to us and they were actually smoking throughout the flight," said Roos, who didn't immediately reply to a message sent to her via Facebook. The second pilot on the 2011 flight was not identified
Malaysia Airlines said it took the allegations very seriously, which it said it was not able to confirm, adding: "We are in the midst of a crisis, and we do not want our attention to be diverted."
Also Tuesday, Malaysian and international police authorities said two people who boarded Flight MH370 with stolen passports were Iranians who had bought tickets to Europe, where they planning to migrate. Their presence on the flight had raised speculation of a possible terrorist link.
Malaysian police chief Khalid Abu Bakar said investigators had determined one was a 19-year-old Iranian, Pouria Nourmohammadi Mehrdad. "We believe he is not likely to be a member of any terrorist group," Khalid said.
Interpol identified the second man as Seyed Mohammed Reza Delavar, a 29-year-old Iranian, and released an image of the two boarding at the same time. Interpol Secretary General Ronald K. Noble said the two men traveled to Malaysia on their Iranian passports, then apparently switched to their stolen Austrian and Italian documents.
CIA Director John Brennan said in Washington that Malaysian authorities "are looking very carefully at what went wrong; you know, if these individuals got onto the plane with these stolen passports, why they were not aware of it."
He also said there has been "a lot of speculation right now — some claims of responsibility that have not been, you know, confirmed or corroborated at all. We are looking at it very carefully."
Asked if terrorism could be ruled out, Brennan replied, "No, I wouldn't rule it out. Not at all."
The United States has sent two Navy ships, at least one of which is equipped with helicopters, and a Navy P-3C Orion plane that can detect small debris in the water. It said the Malaysian government had done a "tremendous job" organizing the land and sea effort.
Vietnamese planes and ships also are a major component of the effort.
Lt. Gen. Vo Van Tuan, deputy chief of staff of Vietnamese People's Army, said authorities on land had also been ordered to search for the plane, which could have crashed into mountains or jungle. He said military units near the border with Laos and Cambodia had been instructed to search their regions.
"So far we have found no signs ... so we must widen our search on land," he said.
Associated Press writers Tran Van Minh in Hanoi, Vietnam, Jim Gomez and Chris Brummitt in Kuala Lumpur, Christopher Bodeen in Beijing, Eileen Sullivan in Washington and Airlines Writer Scott Mayerowitz in New York contributed to this report.
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