Stress or age may have caused a plaster ceiling to collapse at a middle school Monday, injuring 28 students in a shower of debris and heavy dust as they were eating lunch, the school superintendent said.

"I don't know whether we'll ever be able to determine" what caused the collapse, Sioux City School Superintendent Dr. Thomas Brown said. "The engineer's looking at the structural integrity of the roof, and it doesn't appear that there was any moisture. It may be that it was just age and stress. It may be that it just wore out."A 25-foot-by-25-foot section of plaster and a light fixture dropped without warning from the ceiling of a basement cafeteria at West Middle School about 1 p.m., Brown said. The debris hit students on their heads and backs and choked them with a heavy dust.