A police corruption case involving fraudulent sniffer dog tests, which led to hundreds of people being falsely incriminated, has a longer history than previously believed, the Volkskrant reports.

In 2006 the public prosecution office ruled all 2,685 tests carried out by the Oost Nederland police force since 1997 were invalid after a case against two muggers collapsed.

The case fell apart after two police officers admitted the test identifying them had not been carried out ‘blind’, which means the police officers knew which sample contained the scent of the suspect.

According to experts cited in the paper, the dog’s choice would have been informed by that knowledge.

The Volkskrant has now found that dog handlers in the Oost Nederland force had been tampering with sniffer tests in the years before 1997. The paper bases its claim on a report by the WODC research centre published in 1998, which had remained under the radar until now.

That report, which looked at 700 pre-1997 cases, found that the dogs in the Oost Nederland police force identified the suspect in 74% of cases, compared to 51% in police forces elsewhere in the country. One dog handler even booked an 86% success rate.

The public prosecution department was unable to say why nothing was done with the WODC report or why 1997 was given as the cut-off point, the VK reported.