Technicians found sandpaper, wire and tiny bits of debris inside a space shuttle external fuel tank scheduled for use in the third post-Challenger mission, officials at Florida's Kennedy Space Center said. A NASA spokeswoman said the debris, apparently sealed inside the tank during assembly, did not pose a safety threat to the mission but nonetheless was a "cause for concern." Officials said the debris was found last week by technicians working inside the huge tank scheduled for use with the shuttle Columbia in December, 1988. The rust-colored tanks are built by Martin Marietta Corp. at a plant near New Orleans. A company spokesman said the debris has been shipped back to the plant for analysis.