Almost 100 people were injured in the explosion, including the student whose involvement the police are now investigating.
Of the injured, 29 people are still being treated in hospital, two of whom are in intensive care.
Police say they have collected "several pieces of evidence," including gunpowder that may have caused the explosion. The motive is still unclear and at this point, only the student is a suspect, but police do not rule out that more people may be involved.
