Prisoners across the Netherlands are to be issued with calendars for their cells featuring unsolved murders or disappearances as part of a drive by the Dutch police to crack unsolved cases.

The so-called cold case calendars will be handed to all 30,000 prisoners in the country after a trial run in five jails in the north resulted in 160 tips to the police.

Each week of the year in the brightly coloured 2018-19 calendars will be illustrated with a photograph of a missing person and details of the case. The hope is that many of those in jail will know details of some of the crimes or may have heard other criminals chatting about them.

Following the trial, the police received tipoffs on several cases, including that of Nicky Verstappen, an 11-year-old boy who was murdered while at a summer camp near Brunssum in the south of the Netherlands in August 1998. Nobody has ever been convicted, but the police have now reopened the case.

Jeroen Hammer, the calendar’s inventor, told Dutch newspapers the calendars had also proved popular with bored prisoners, although some had regarded the initiative as an attempt to turn them against their own. “Of course there are people who don’t want a calendar and who don’t want to be seen as a snitch,” he said. “But our trial shows that almost two-thirds of prisoners think the calendar is a good idea.”

Two other cases on the calendar have also been reopened following tipoffs this year from inmates in prisons in Leeuwarden, Zutphen, Sittard, Zwaag and Schiphol. The calendar has been printed in Dutch, Arabic, Spanish, English and Russian to maximise its impact, and a €800,000 reward is being made available for those whose information ends in a successful conviction.

A promotion video for the cold case calendar

Announcing the national rollout of the calendar, a police spokesman said: “The experience of the police and judiciary shows that prisoners have relatively high levels of knowledge about committed crimes.”

On their website the Dutch police said advances in forensic technology had made acting on tipoffs even in very old cases significantly easier. There are around 1,500 unsolved crimes classified as cold cases in the Netherlands.

“There are two important factors that play a role in this,” the police said. “On the one hand there are developments in forensics, which offer the teams more opportunities, such as DNA. On the other hand, witnesses can play a crucial role. Witnesses who had seen something at the time, but did not dare to come forward, sometimes dare to do so over time. Dutch research shows that there are also people who have heard something about an unresolved issue at a later date.”

The police say they can offer anonymity to people in certain cases.

“There is no penalty for keeping information about a criminal offence committed. Therefore, you do not have to fear persecution if you have been sharing information, even after years of deliberation,” they said.

The idea of the calendars was borrowed from the United States, where every year several states distribute a deck of cards containing information about cold cases among prisoners.