Twelve police armed with submachine guns raided a house less than 20km away, arresting a man in his forties, in an operation police confirmed was linked to the explosives discovery.
But police removed the cordons in Rygge on Wednesday afternoon after discovering that boxes found outside a house and labeled as dynamite were in fact empty.
The 56 students at Våk nursery school in the town of Våler i Østfold were evacuated at 12.30pm on Wednesday. Meanwhile, the 180 students at the next-door primary school were asked to go on foot to a nearby shop, where they could wait for their parents.
A specialist bomb disposal unit was sent from Oslo to defuse the would-be explosives.