The police have received 56 tip-offs since Tuesday evening’s television crime show report about the recent spate of letter bombs, two of which have actually gone off but caused no injuries.

During the programme, police revealed more details about the letter bombs, which are always signed with a distinctive row of dashes. The bomb itself uses a 9 volt industrial battery by Duracell and is packed in a paper bag featuring tourist scenes of Delft.

So far, 10 different companies, ranging from petrol stations to banks and hotels, have received the threats, and the justice ministry has issued a €15,000 reward for information leading to an arrest.

The blackmailer is demanding payment in bitcoin and each threat involves a different amount of money. Seven of the letters were sent in early January and a further three last week.