Malaysian officials met in Beijing with several hundred Chinese relatives of passengers to explain the search and investigation, and to relay the last transmission that Malaysian air traffic controllers received before the plane entered Vietnamese airspace, according to a participant in the meeting.
Aviation officials in Vietnam said they never heard from the plane.
Its sudden disappearance led to initial speculation of a catastrophic incident that caused it to disintegrate. Another possibility is that it continued to fly despite a failure of its electrical systems, which could have knocked out communications, including transponders that enable the plane to be identified by commercial radar.
Authorities have not ruled out any possible cause, including mechanical failure, pilot error, sabotage and terrorism, and they are waiting to find any wreckage or debris to determine what went wrong.
In June 2013, Boeing issued a safety alert to Boeing 777 operators, telling them to inspect for corrosion and cracks in the crown fuselage around a satellite antenna. The alert says one airline found a 16-inch crack in one plane, then checked other 777s and found more cracking.
"Cracks in the fuselage skin that are not found and repaired can propagate to the point where the fuselage skin structure cannot sustain limit load," Boeing said. "When the fuselage skin cannot sustain limit load, this can result in possible rapid decompression and loss of structural integrity."
Two U.S. Federal Aviation Administration technical experts and a regional representative are in Kuala Lumpur as part of an NTSB team supporting the investigation. Experts in air traffic control and radar are providing technical help, the board said.
Hishammuddin described the multinational search as unprecedented. Some 43 ships and 39 aircraft from at least eight nations were scouring an area to the east and west of Peninsular Malaysia.
"It's not something that is easy. We are looking at so many vessels and aircraft, so many countries to coordinate, and a vast area for us to search," he told a news conference. "But we will never give up. This we owe to the families of those on board."