THE HAGUE, Netherlands (AP) — A letter bomb that detonated Wednesday in an Amsterdam mail sorting center was sent by an extortionist who is demanding payment in bitcoins, police said. Nobody was injured when the package went off in the mail sorting office.

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.