A half hour later, another letter bomb went off in the southern city of Kerkrade, also without injuring anybody. In both cases, police said that staff reported a hiss when the letter was opened followed by a bang.
Amsterdam police said in a statement that they are taking into account the possibility that the letters are linked to a string of letter bombs and warning letters sent to businesses last month. None of those devices detonated.
There have been no arrests announced in the investigation.