The EU Criminal Intelligence Agency has reported that at least 10,000 refugee children have disappeared after arriving on European soil, The Guardian reported. These children were unaccompanied by any adults, and they seem to have vanished after registering with local authorities.

Europol Chief of Staff Brian Donald gave a statement about these worrying disappearances.

"It's not unreasonable to say that we're looking at 10,000-plus children. Not all of them will be criminally exploited; some might have been passed on to family members. We just don't know where they are, what they're doing or whom they are with," he said, according to RT. "An entire [criminal] infrastructure has developed over the past 18 months around exploiting the migrant flow. There are prisons in Germany and Hungary where the vast majority of people arrested and placed there are in relation to criminal activity surrounding the migrant crisis." 

Officials are urging people to keep an eye out for these migrant children, as they are the most vulnerable, especially with the rise in human trafficking that has followed the refugee crises.

And while authorities have reported 10,000 missing children, its more likely that the number is much greater than that.

"Whether they are registered or not, we're talking about 270,000 children. Not all of those are unaccompanied, but we also have evidence that a large proportion might be," Donald said.

Italian officials said in May 2015 that nearly 5,000 children disappeared from asylum centers, BBC News reported. Sweden saw 1,000 refugees disappear in October.

This is a growing problem, and the warning is aimed at helping these numbers drop, though it'll be a long and tough process for European officials.