A 51-year-old man accused of blackmailing supermarket chain Jumbo and placing explosives at two branches has been jailed for eight years by judges in Groningen.

Alex O was traced by police via two kitchen timers which he had left with explosives at two Jumbo stores in Groningen last May. The timers were both bought at Blokker stores in the city.

O, who denies the charges, was also found guilty of sending a letter containing explosives to a branch of Jumbo in Zwolle. He was identified from dna under the stamp.

O, who the court was told had sent 15 threatening letters to Jumbo, demanded payment in bitcoins – a virtual currency traded on the internet. Transactions are difficult to trace, allowing people to remain anonymous.

According to news agency ANP, he had demanded 2,000 bitcoins, which were worth around €1.3m at the time.

O denies the charges and after the hearing, family members shouted from the public gallery that ‘there will be an appeal and then we’ll get them’, the Telegraaf reported.