A 50-year-old man and a 15-year-old boy from Groningen have been arrested for attempting to extort money from the Jumbo supermarket chain.

The police investigation centred on three incidents involving explosives and bomb threats at Jumbo stores in Groningen and Zwolle, the last of which took place on Friday, the public prosecution department said in a statement.

Both explosive devices involved a kitchen timer which was traced to a Blokker store in Groningen. That evidence, plus tips from the public, led police to make the arrest.

The man had also threatened Jumbo and made bomb warnings on other occasions, the department said. He had demanded payment in the digital currency bitcoins.

According to local broadcaster RTV Noord, the man is Alex O, who was sentenced to 14 years in jail in 1993 for his role in a string of violent robberies in the north of the country. He is also said to have been involved in soft drugs.

Local media say the 15 year old is O’s son. Both have been remanded in custody.