The team investigating the Columbia space shuttle disaster say they have found why it disintegrated in mid-air.
After a simulation test, investigators said they had proof that a piece of foam insulation hitting the orbiter's left wing led to its destruction.
A Nasa investigator examines the hole after the simulation test
Scott Hubbard, a member of the Columbia Accident Investigation Board, said the test carried out on Monday showed that this was "the most probable cause" of the shuttle's break-up when it had entered the atmosphere.
In the test, a piece of foam was fired at high speeds into a wing panel of a similar shuttle, blowing open a large hole.
"We have found the smoking gun," Mr Hubbard said after the test at the Southwest Investigation Institute in San Antonio.
In the test, a 0.8-kilogram piece of foam was fired by a 10.6-metre nitrogen-powered gun into the wing panel from the orbiter Atlantis at about 850 km (530 miles) hour, blowing out a 40-centimetre square hole.
The impact was so violent that it popped a lens off one of the cameras recording the experiment and prompted gasps from about 100-strong astonished crowd.
"It was a visceral reaction," Mr Hubbard said.
Columbia's left wing was fatally damaged during lift-off
He also said he believed that repairing the damage to the wing while the Columbia was on orbit would have been a nearly impossible task.
He said that the damage to the shuttle's wing would have pierced its protective shield, causing its break-up.
Columbia disintegrated on re-entry on 1 February over Texas, killing all seven of its crew and scattering debris across several US states.
The investigators had previously indicated that the piece of foam from external fuel tank which struck Columbia's left wing 82 seconds after the orbiter began its lift-off was the most likely cause of the disaster.