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Steel wires are believed to have failed before ducting collapsed at a college hall as pupils sat exams.
Those are the interim findings of an report into the accident at a gym in Minster College on May 12.
A statement by investigators reveals initial findings have shown the steel wires which attached the ducting to the hall roof failed.
Evidence gathered from the hall will now be examined at a laboratory in Derbyshire.
The statement continues: "It would be inappropriate to speculate, at this stage, on what may have caused the failure of the steel wires. "
The full investigation is likely to take some time.