NASA's investigation into the Columbia disaster is now wide open. The space agency says there's just not enough evidence to back the theory that damage to the shuttle's protective heat tiles caused the crash. At today's memorial service for the Columbia astronauts at the National Cathedral in Washington, D.C., NASA chief Sean O'Keefe pledged the agency would find the cause of the accident and continue with space exploration. NPR's Richard Harris reports on the progress of the investigation.
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