Space Center, Houston ? A day after all but ruling it out as a leading cause, NASA said Thursday that investigators still were considering whether a piece of insulating foam that struck Columbia’s wing during liftoff was enough to bring down the shuttle.

Shuttle program manager Ron Dittemore said that even though the possibility appeared remote, investigators must remain open to every option as they put together a so-called fault tree into what caused Columbia’s fiery breakup just minutes from its landing Saturday.

“The foam that shed off the tank and impacted the left wing is just one branch, and we are pursuing that,” he said. “Even though we scratch our heads, we’re going to pursue it and we’re going to pound it flat.”

Eighty-one seconds into launch, a 2 1/2-pound, 20-inch chunk of foam from Columbia’s external fuel tank broke off and slammed into the underside of the shuttle’s left wing.

The accident investigation board, led by retired Navy Adm. Harold Gehman Jr., arrived Thursday at Johnson Space Center and met with Dittemore and other shuttle officials. NASA Administrator Sean O’Keefe pledged from Washington that “every single piece of evidence, every fact, every issue” would be checked, and the board’s conclusions would be final and absolute.

Before ruling out the foam as a culprit, NASA will be testing its impact on the thousands of fragile thermal tiles that cover each space shuttle. In addition, the analysis that was conducted during Columbia’s flight is being redone “to see if there was anything that we missed,” he said.

Dittemore said the camera views of the flyaway foam during liftoff Jan. 16 could have been better. “It’s a disappointment that the camera with the very best view turned out to be out of focus,” he said. “We’re just going to have to live with what we have.”

Engineers taking part in NASA’s so-called reverse analysis struggled Thursday to make sense of the eight minutes recorded between the time the first sign of trouble appeared aboard Columbia over California and the shuttle’s final, dying moment over Texas.