The Boeing 777 was flying from Kuala Lumpur to Beijing when it vanished early Saturday, less than an hour after takeoff, without sending a distress signal.
Turning off the transponder would make the aircraft unidentfiable to civilian controllers, but it would remain visible to the type of radar used by militaries.
No apparent terror link
The head of Interpol says the jet's disappearance does not appear to be related to terrorism. However, the director of the U.S. Central Intelligence Agency said Tuesday that terrorism could not be ruled out.
"You cannot discount any theory,"' CIA Director John Brennan said in Washington.
Interpol Secretary-General Ronald Noble says new information about two Iranian men who used stolen passports to board the plane makes terrorism a less likely explanation for the jet's disappearance.
The international police agency released photos showing the two boarding the plane at the same time. They are identified as Pouri Nourmohammadi, 19, and Delavar Seyedmohammaderza, 29.
Malaysian Police Inspector General Khalid Tan Sri says the 19 year old was likely trying to migrate to Germany.
"We have been checking his background. We have also checked him with other police organizations on his profile, and we believe that he is not likely to be a member of any terrorist group," the inspector told reporters. "And we believe that he is trying to migrate to Germany."
Khalid said Nourmohammadi's mother knew he was traveling on a stolen passport.
The other man's identity is still under investigation. But the development reduces the likelihood they were working together as part of a terror plot.
Meanwhile, an extensive review of all of those on board continues.
?Khalid says authorities are looking into four possible scenarios in connection with the plane's disappearance: hijacking, sabotage, personal disputes and the psychological condition of those on board.
"There may be somebody on the flight who has bought huge sums of insurance. Who wants the family to gain from it. Or somebody who owes so much money and you know," he said, adding that they are looking at all possibilities.
Air Malaysia says it is in negotiations regarding financial aid with relatives of the Chinese passengers on board.
Meanwhile, Malaysia Airlines is also looking into an Australian television report that the co-pilot of the missing plane once invited two women into the cockpit during a flight.
Jonti Roos said she and her friend stayed in the cockpit during the one-hour flight on Dec. 14, 2011, from Phuket, Thailand, to Kuala Lumpur. She also said the crew smoked during the flight.
"Malaysia Airlines has become aware of the allegations being made against First Officer Fariq Ab Hamid which we take very seriously. We are shocked by these allegations. We have not been able to confirm the validity of the pictures and videos of the alleged incident,'' the airline said.
Some information in this report was provided by Reuters news agency and VOA's William Ide in Beijing.