Story highlights Europol: Some of those missing might be vulnerable to human trafficking

More than 1 million migrants entered Europe last year, many escaping war or persecution

(CNN) About 10,000 unaccompanied migrant children who traveled to Europe may be unaccounted for, the European Union's police agency said.

The estimate is based on reporting from EU member states, nongovernmental organizations and other international groups, Europol spokesman Jan Op Gen Oorth said.

Europol said the names of some suspected migrant smugglers are also appearing in the agency's human trafficking database -- indicating some of those missing might be vulnerable to exploitation.

"Not all of them will be criminally exploited. Some might have been passed onto family members, for example," Op Gen Oorth said. "Europol's concern is that we do not know where they are or what they're doing or who they are with. This is a worrying underlying issue in relation to the migrant crisis."

Leonard Doyle, spokesman for the International Organization for Migration, said the estimated figure appalled him.