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Najib, who was premier when MH370 vanished with 239 people on board in 2014, was responding to remarks by former Australian Prime Minister Tony Abbott that Malaysian leaders had considered from the outset that flight captain Zaharie Ahmad Shah may have committed mass murder.
Najib told online news portal Free Malaysia Today that Malaysian officials had considered such a scenario during their investigation but had chosen not to make their views public.
"It would have been deemed unfair and legally irresponsible since the black boxes and cockpit voice recorders had not been found and hence, there was no conclusive proof whether the pilot was solely or jointly responsible,” Najib was quoted as saying.
"Again I must stress that this possible scenario was never ruled out during the search effort and investigations, where no effort was spared."
A spokesman for Najib confirmed his remarks.
Najib said there were several reasons for authorities to suspect Zaharie's involvement, including his ownership of a home flight simulator and findings showing that MH370's transponders were switched off shortly after the plane left Malaysian airspace.
Flight MH370 became one of the world's greatest aviation mysteries when it disappeared on its way from Kuala Lumpur to Beijing on March 8, 2014.
Malaysia, China, and Australia, called off a two-year, $A200 million ($NZ209 million) underwater search in the southern Indian Ocean in January 2017 after finding no trace of the aircraft.
A second three-month search, led by US firm Ocean Infinity, ended similarly in May 2018.
Najib lost a general election that month. He is now facing dozens of corruption charges over alleged involvement in a multibillion-dollar scandal at a state fund in which he has plead not guilty.