Houston ? NASA investigators looking into what caused large pieces of foam to break free from Columbia and Discovery have found nine small cracks in the foam of an external fuel tank.

But program administrators are not sure whether the cracks played any role in the foam separation from the space shuttles.

“Certainly, the cracks we found … are interesting,” said John Chapman, who is responsible for the production of NASA’s external tanks. “We’re still in the process of understanding what that means.”

The shuttle fleet remains grounded while NASA figures out how to stop big pieces of foam insulation from breaking off the fuel tanks during liftoffs, a problem that led to Columbia’s destruction in 2003 and reappeared during Discovery’s July launch.

Wayne Hale, deputy shuttle program manager, said the agency is still working toward launching another shuttle next May, but won’t let an arbitrary schedule dictate when the agency flies again.