Two letter bombs exploded at two separate businesses in the Netherlands on Wednesday.
The police said the attack was blamed on extortionists who had demanded payment in bitcoin. Nobody was injured in the attacks.
One explosive detonated at an ABN Amro bank mail-sorting office in Amsterdam and another in a mailroom of Japanese electronics group Ricoh.
The authorities said it is likely that other letters were sent out to other locations.
An employee at the Amsterdam sorting office threw a letter away after hearing hissing noises. The small bomb exploded on impact.
The police have been investigating multiple incidents of letter bombs since January 3. All the letter bombs, believed to have been sent by one person, were intercepted before they could go off.