About the size of a suitcase, "the foam is the most probable cause of the accident," Admiral Harold Gehman, who led the commission named to probe the catastrophe, told a news conference.

Theories blaming damage to the shuttle's crucial heat shield on the errant piece of foam have been rife for months, but Tuesday's statement was the most definitive yet on the cause of the disaster.

Investigators previously had considered the foam, which separates from the central fuel tank minutes after launch, as one possible cause.

Gehman said US shuttles could be back in flight in nine months or sooner.

"I dont speculate on the date of return to flight," he said.

"I would say that having read every word of the draft report and having gone over what might be possible recommendations I don't see any recommendations which are so difficult to accomplish that they shouldn't be able to return to flight in six to nine months."

Commission member Roger Tretault said a fastidious analysis of the tons of Columbia debris recovered from a vast area of Texas and Louisiana resulted in a "strong indication" that a part of the shuttle's left wing had been pierced during takeoff by a piece of insulation torn from the external fuel tank.

The shuttle's underbelly, nose and forward wing surfaces are protected by the thermal tiles from the enormous friction heat generated when the shuttle re-enters the Earth's atmosphere.

Gehman listed four conditions NASA should insist upon before considering resumption of shuttle flights.

Firstly, he said, is to "minimize or prevent" chances of another detachment of fuel tank insulation on blastoff.

Secondly, the shuttles themselves must be reinforced to better withstand the shock of such an impact from insulation or other debris involved in blastoffs.

Thirdly, NASA must its ability to affect post-takeoff inspection and, if necessary, to undertake emergency repair missions while in orbit.

And fourthly, said Gehman, NASA should work on improving its crew survival and rescue procedures.

The commission is to submit its report on the accident to NASA at the end of July. But NASA officials, who have been kept up to speed on the progress of the commission's investigation, have said work is already in progress to implement the recommendations.

A NASA report released last month said Columbia could not have been saved once it began its return to Earth, not even by reducing its weight or changing its trajectory.

The April 22 internal report from the team led by NASA flight director LeRoy Cain examined three options to reduce the shuttle's weight by as much as 16 tonnes, and concluded none would have reduced the extreme heat of re-entry enough to save the doomed spacecraft.

The report also noted the extreme risk of shedding unnecessary equipment, such as scientific experiments, and said it would only be justified by "significant and convincing data" proving the shuttle would not survive re-entry.

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