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 science mission that did not include a visit 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.

The board has performed six previous tests. The foam caused far less damage to the panel. However, one block of foam did crack the carbon coating while another made a crack the size of a dime where two of the panels connect.

However, Monday's test was the first convincing proof that the foam could breach the "carbon-carbon" sheeting.

"I felt surprised at how it appeared, such a dramatic punch-through," Hubbard said.

"But it is the ... type of damage that must have occurred to bring down the orbiter."

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 Monday's blast means the panel will have to work harder to find a solution.

A former professor of aerospace engineering at St. Louis University said the damage to Columbia's wing could have been fairly small. If it had been large, the Columbia would have broken up even earlier.

"Once you had a crack through the leading edge, all the way through like the second test, it was going to fail," Paul Czysz, said.

"All you're arguing over now is it going to burn up over Los Angeles, over New Mexico or over Texas."

The hardened material on the leading edges of the wings wards off gasses as hot as 1,600 degrees C (3,000 degrees F) and is also on the shuttle's distinctive black nose cone.

The material is not designed to absorb impact, let alone the large chunk of foam that hit Columbia 82 seconds after liftoff.

Although smaller pieces have broken loose previously on liftoff, the piece that hit the shuttle was unusually large.

"That confirms our worst fears, that when (NASA) did not tend to the fact of that foam coming off, they did not understand the engineering and design of those ... panels, and therefore, let slip what is to be the cause of the disaster," said Florida Senator Bill Nelson who flew aboard Columbia in 1986 and sits on the Senate committee that oversees NASA.

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