"The doors of six sampling sites are open!" the police force in the southern Limburg province said on Twitter.
Around 21,500 males aged between 18 and 75 now have three weeks to voluntarily submit their DNA for testing as part of the reopened probe into the murder of Nicky Verstappen, 11, in 1998.
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Verstappen was last seen alive at a camp in a nature reserve near the border with Germany on the night of 9 August that year. His body was later found close by.
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Dutch police said the men asked to volunteer DNA were not suspects in the case, stressing that the operation was primarily aimed at pinpointing if any donor is related to an unknown person who left traces of their DNA on Verstappen.
Analysis of the samples is expected to take between six months and a year.
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