The police were alerted that a masked person had been seen in a stairwell in central Nyköping and disappeared on the road in a car, which the police were able to locate after a short search.
In the car was the man in his 20s and a boy in his 15s, as well as a suspected dangerous object. The two people were arrested, suspected of preparing for public destruction.
The National Bomb Squad was called but judged the object to be harmless.
The arrest of the younger boy was lifted while the man in his 20s is now in custody.