"These children are some of the most vulnerable, and many become victims of violent crime or sexual abuse along the dangerous journey," the White House said in a statement on June 20, 2014, during the height of the influx. 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.
Europe 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. Targeting refugees
"It's not unreasonable to say that we're looking at 10,000-plus children," Europol's chief of staff told the Observer newspaper.
"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."
Officials in Italy warned in May 2015 that almost 5,000 children had disappeared from asylum reception centres since the previous summer.
