The Jumbo supermarket chain and the police have introduced extra security measures at all 579 stores following a series of bomb alerts.
Jumbo is being blackmailed by an unknown person who is demanding a ransom in bitcoins. The police will not say how much money is being demanded.
The bitcoin is a virtual currency traded on the internet. Transactions are difficult to trace, allowing people to remain anonymous.
The blackmailer, who first appeared in May, has already left explosives at two Jumbo stores in Groningen, one of which exploded. A window was damaged but no one was hurt.
Another store was evacuated following a bomb alert but nothing was found. And last month a branch in Zwolle was sent an envelop containing explosive material.
On Thursday, the police launched a website asking for information. ‘We have a lot of the pieces of this puzzle but a seemingly unimportant detail from a citizen could be the final piece that completes the puzzle,’ Rob Vos, who is leading the investigation, told news agency ANP.
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