Admiral Hal Gehman, who heads the Columbia Accident Investigation Board, said in a statement: "We now know enough ... to the point where we should now focus our effort."

He described the preliminary findings as "a working hypothesis."

Columbia broke up February 1, killing all seven astronauts on board, just 16 minutes prior to its scheduled touchdown in Florida.

Investigators believe heat-resistant tiles on the shuttle's left wing were damaged at liftoff by pieces of insulating foam that broke off the shuttle's external fuel tank.

Superheated gases in the atmosphere entered the shuttles skin and burned up the spacecraft.

SPACE.WIRE