Police say they are ‘satisfied’ with the response to a call for DNA samples to help them solve the murder of an 11-year-old boy in 1998, even though one in three people declined.

Altogether more than 21,000 men in Limburg were asked to submit their DNA during the last three weeks in and around Heerlen, Brunssum and Heibloem, making it the largest mass sampling exercise ever carried out in the Netherlands.

Police hope that the exercise will enable them to trace the killer of Nicky Verstappen, who was found dead on the Brunsummerheide heathland during a summer camp 20 years ago, by identifying relatives. The response rate of 65.3% was lower than in other DNA sampling exercises carried out in connection with unsolved murders.

Project leader Hans Ramaekers said: ‘Of course the chance of succeeding is highest if you have a 100% attendance rate. All our efforts were geared towards achieving that. But even at this level we may find we have enough DNA profiles from all branches of the family tree to do a thorough investigation.’

In 2012 an appeal for DNA samples in connection with the murder of 16-year-old Marianne Vaatstra, in Friesland, in 1999 yielded an 89% response rate. The killer, Jasper S., gave a sample which led directly to his arrest.

Last year 125 out of 133 men of Turkish origin in Zaandam who were invited to give samples to help solve the killing of Milica van Doorn submitted their DNA. Only two refused, one of whom, 47-year-old Hüseiyn A., was later arrested for the murder.