Investigators looking into what caused the U.S. space shuttle Columbia to disintegrate as it returned to earth last February say evidence uncovered so far supports the theory that the spacecraft was damaged during lift off. They believe a piece of insulation foam broke loose from an external fuel tank. But more tests still need to be done before all other possible causes can be ruled out.

The independent board investigating the Columbia disaster says most of the information collected so far tends to support early theories that a piece of insulating foam struck Columbia's left wing during lift off.

But the board's chairman Admiral Harold Gehman says more tests of how the foam might have collided with the shuttle still need to be done before any final conclusions are drawn.

"We're careful not to say that the foam knocked a hole in the leading edge of the orbiter because we can't prove it," he said. "Now, that's not to say that we don't believe that's what happened."

But after three months of investigating, the panel is now working on the theory that damage to the shuttle wing's heat resistant tiles during lift off, first dismissed by NASA as insignificant, may have ended up setting off the chain of events that caused the spacecraft to break up. All seven astronauts on board were killed.