- 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
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.
"We think it's shocking that we are now learning that there are so many unaccompanied minors exposed to trafficking and other dangers," Doyle said.
"That is another tragic twist in the latest story of migration to Europe and the need to protect vulnerable young people who find themselves at loose without friends in Europe and therefore vulnerable without proper mentoring and leadership."
More than 1 million migrants entered Europe
last year. Many are refugees from Syria, where civilians grapple not only with a civil war but also the threat of ISIS. Others came from elsewhere in the Middle East and North Africa, trying to escape poverty and persecution.
But thousands who attempted the journey across the Mediterranean Sea didn't make it.
At least 3,695 either drowned or disappeared last year as they attempted to cross the sea on unseaworthy boats, according to International Organization for Migration figures. That's a rate of about 10 deaths or disappearances a day.
While some countries are obligated to take in refugees
, the massive influx in Europe -- combined with crimes blamed on migrants -- has caused some countries to take a harder stance on migrants.
Denmark, for example, recently adopted a controversial law to seize asylum-seekers' valuables to help cover their expenses