Police in the Netherlands are appealing for witnesses to find a person who is planting explosives and requesting a bitcoin ransom.

According to the police, the first attacks took place in May, when the perpetrator placed explosive devices in three different Jumbo supermarkets – a chain of stores belonging to the Van Eerd Group – in Groningen, the largest city in the north of the country.

Subsequent attacks followed in June and July when the offender sent a congratulatory card containing a small amount of explosive substance to another supermarket in Zwolle, a small town in the province of Overijssel, also in the north of the Netherlands.