More than 10,000 migrant children may have disappeared after arriving in Europe over the past two years, the EU's police intelligence unit says.
Europol said thousands of vulnerable minors had vanished after registering with state authorities.
It warned of children and young people being forced into sexual exploitation and slavery by criminal gangs.
Save the Children says some 26,000 child migrants arrived in Europe last year without any family.
It is the first time Europol has given a Europe-wide estimate of how many might be missing.