Blockchain: The revolution that hasn't quite happened Two months of attacks
The letter-bomb campaign began in January, with a series of packages carrying the logo of a company in Rotterdam that specialises in debt collection. Police have stressed that the company, CIB, is not related to the attacks.
Letter bombs have since targeted companies across the Netherlands, including Rotterdam, Utrecht and Maastricht.
But for almost a month the attacks then stopped.
Two packages then blew up in the mailrooms of two firms in Amsterdam and Kerkrade on Wednesday, although this time neither bore the CIB logo.
The first was at ABN Amro, in the west of the capital, when a worker described opening a letter that started making a hissing sound. He threw it away and it made a bang, police said. Image copyright EPA Image caption Forensic scientists have been trying to find DNA evidence from the letter bombs
The second went off around half an hour later in the mail room of a firm that supplies office products in the southern town of Kerkrade.
The first attack on Thursday morning targeted a financial services company called Unisys in Leusden in the centre of the country.
Not all the targets have been financial services firms. Hotels, sorting offices, a filling station, a car dealership and an estate agency have also been hit.
The device in Leusden did not explode and the Dutch bomb disposal unit dealt with the package, whereas the second letter bomb at ING's headquarters did.
Police said they had yet to analyse the ING bomb but all the others included blackmail letters demanding payment in the cryptocurrency Bitcoin.
A spokeswoman said details of the letters were not being made public nor were the results of forensic tests on the letters.
Police say for now they are investigating all scenarios. Related Topics...