In a test, investigators fired a 1.67-pound (0.75-kilo) chunk of the foam at a panel taken from another shuttle's wing.

A 10.6-meter (35-foot) nitrogen-pressurized gun sent the suitcase-sized piece of foam hurtling into the wing panel at 800 kilometers (500 miles) per hour, opening a 40-centimeter (16-inch) hole with a blast so strong that it broke one of the gauges tracking the experiment.

"We have found the smoking gun," Columbia Accident Investigation Board member Scott Hubbard said.

"This is in fact the most probable cause creating the breach that led to the accident of the Columbia, the loss of crew and vehicle."

Hubbard also said he believed repairing the damage to the wing while the shuttle orbited the Earth would have been nearly impossible.

Columbia disintegrated over the US state of Texas February 1 as it was headed for landing at Cape Canaveral, Florida, after a mission to the International Space Station. All seven astronauts aboard died.

A piece of foam separated from the external fuel tank after launch and damaged the left wing of the craft.

After the foam was fired, making a hole in the wing panel, reporters and officials on hand let out an audible gasp.

"It was a visceral reaction," Hubbard said.

The damage allowed hot gases to pierce the shuttle's protective shield during descent and ultimately caused its disintegration, according to investigators.

Having established a cause makes it more likely that the board will be able to complete its report and to submit it to Congress by the end of July. However, whether NASA will get the shuttles flying by early next year as it hopes is still an open matter. Much depends on Congress and the White House.

The board has already recommended finding a way to repair such damage once the shuttle is in space to avoid another deadly re-entry.

However, the size of the hole produced by the blast Monday means the panel will have to work harder to find a solution.

SPACE.WIRE