HOUSTON – Columbia accident investigators announced Tuesday they have concluded that damaged thermal tiles allowed superheated gases inside the space shuttle’s left wing as it re-entered the atmosphere, leading to its destruction.

“We have a very good understanding of what happened,” investigation board chairman Harold Gehman said at a news conference. “The orbiter (space shuttle) was returning with a pre-existing flaw in the wing…. The wing got heated from the inside.”

However, Gehman said how the wing was damaged remains unproven, although a collision with foam insulation from the shuttle’s external fuel tank during launch remains the most likely cause.

The shuttle came apart over Texas on Feb. 1 while streaking toward a planned landing at the Kennedy Space Center in Florida. Seven astronauts were killed and the shuttle fleet was grounded while investigators looked for the cause of the accident.

The board for weeks was unwilling to take a firm position on what happened even as evidence mounted that a hole in the left wing was at the heart of the disaster. Even Tuesday, the board called its conclusions a “working scenario.”

But Gehman said “we made sure that there were no facts that contradicted our scenario.”

“We now know enough,” he said. “We’re at the point where we should focus our efforts.”

He said the board will be able to make a broad range of recommendations this summer to make the shuttle program safer.

Gehman said the board may never be able to prove the wing was damaged by the foam insulation, though technicians next month will test that idea by firing chunks of the material at thermal tiles. But Gehman said NASA already knows peeling insulation is a problem that must be fixed before the shuttle fleet flies again.

Also among the board’s conclusions: