Two suspected letter bombs went off on Wednesday in the Dutch cities of Amsterdam and Kerkrade but no one was injured, police have confirmed.
The first explosion was reported shortly before 0800 CET at a postal sorting centre on Bolstoen, in northwestern Amsterdam.
Police said in a statement that one of the employees heard a "hissing sound" emitting from a letter and threw it away. A "slight bang" then occurred.
The letter bomb contained an extortion letter, demanding payment in bitcoins.
There were no injuries and an initial sweep of the building by experts found no other explosives.
A second explosion took place shortly before 0900 CET on Wiebachstraat, in the southeastern city of Kerkrade, around 220 kilometres from Amsterdam.
Nine people were in the building when the small bomb went off but there were no reported injuries.
Amsterdam police are investigating the possibility that the letters are linked to a string of letter bombs and warning letters sent to businesses last month across the Netherlands.
There was no CIB logo found on the letter as in earlier cases.
Police have urged citizens and companies to give special attention to any "white envelopes with a thickening, provided with two stamps and with the addressee on a printed piece of paper in a plasticised part of the envelope".
No arrests have yet been announced.