How borders are drawn and enforced has far-reaching consequences, whether we live on either side of them or halfway across the world.

The number of unaccompanied refugee children reported missing in Germany has doubled in the past six months.

According to figures released by the Federal Criminal Police Office (BKA) to the Neue Osnabrücker Zeitung (link in German), the total number of children who’ve gone missing after their initial registration hit 8,991 in July—up from 4,749 on Jan. 1.

The BKA said the majority of the children who had disappeared from the system were between 14 and 17 years of age. More than 860 were younger than 13.

The Federal Crime Office said there was no “concrete evidence” that some of the missing children have fallen into criminal hands.

A BKA spokesperson told the paper that when unaccompanied children leave their refugee reception centers, they are registered as missing by the police. Refugee children can sometimes be registered as missing when they move to other refugee centers, as often they have no identity papers, or their names are misspelled. In other cases, the young person might leave to join family or acquaintances elsewhere without notifying the police. But there are other, darker possibilities.

“Many are missing because they are trying to reach family or community,” Federica Toscano from Missing Children Europe told Quartz. “But in many cases, they may be abducted, they may have trusted a person who wants to profit from their vulnerability, or they may be a victim of trafficking.” She said it should not be assumed that children go missing because they have a certain plan of their own.

Authorities don’t report on unaccompanied refugee children in the same way as for other missing children, Toscano added. “We know the police don’t follow up on the disappearance of a unaccompanied child as they would on the disappearance of another child, a German child, an Italian or whatever,” she said. “It’s a problem everywhere in Europe, often disappearances are reported too late, or reported just for administrative purposes.”

“The authorities underestimate risks to which children are exposed,” she said.

Save the Children estimated that around 26,000 unaccompanied children entered Europe last year, and Europol warned that they could have been preyed upon by organized trafficking syndicates. The majority of the children reported as missing in Germany are from Afghanistan, Syria, Somalia, Eritrea, Morocco and Algeria.

“Unaccompanied minors from regions of conflict are by far the most vulnerable population; those without parental care that have either been sent by their families to get into Europe first and then get the family over, or have fled with other family members,” Mariyana Berket, of the Organisation for Security and Cooperation in Europe, told the Guardian in January.

Last year, about one million refugees arrived in Germany, completely overwhelming authorities’ capacity to register them and creating huge backlogs.