Outside the train station in Rome, teen migrants sell drugs from school backpacks and trade sex for cash or clothes. In the capital of Sweden, they steal food from supermarkets and sleep on the streets. From makeshift camps along the northern French coast, they try to hop at night onto the backs of moving trucks headed to Britain.

All across Europe, there is a growing shadow population of thousands of underage migrants who are living on their own, without families. They hide silently and in plain sight, rarely noticed in the crowd. Nobody even knows how many of them there are — Europol estimates broadly that at least 10,000 kids have gone missing from shelters or reception centers.

These unaccompanied minors are slipping through the seams of a European system strained to bursting, and they present one of the biggest challenges of the migrant crisis. The fact that accurate numbers are so hard to come by reflects the shortcomings of the bloc's 28 member states in implementing laws and guidelines that are supposed to protect asylum seekers in general and unaccompanied minors in particular. While the problem is not new, the sheer volume of migrants arriving last year has made it acute.

Like adult migrants, minors are flooding into Europe for both security and economic reasons, the AP found in interviews with more than two dozen. The question is where they end up.

Imran, a 13-year-old from Afghanistan, has passed through at least eight countries, mostly on foot, and is now trying to make it from a squalid migrant camp in Calais to the U.K. He dresses neatly in a donated sweat suit and tells his story politely. Only his gym shoes cracking at the seams and the cloud over his face hint at the hardship of his life, light years from his dreams of going to "doctor school."

The Taliban shot Imran's father when he was 8 or 9, and threatened to kill him too when he grew up. He says he started to receive letters at his house. Frightened, his mother sold the building that provided the family's income to pay smugglers for Imran to join his uncle in Britain.

Most nights Imran tries to sneak onto trucks to Britain — a dangerous venture which the Afghans call "going to the game." A 15-year-old Afghan died in January from sticking his head out and hitting an obstacle, and a 7-year-old who escaped Calais almost suffocated last month after the locked truck reached Britain. It is not uncommon here to see migrants with crutches or other injuries from failed efforts to jump onto moving vehicles.

During the day, Imran looks haggard from lack of sleep. He has not talked to his mother in four months, and he describes life in the camp residents dub the Jungle as fit for "dogs and cats." Imran is his nickname; the AP is not using the full names of children in this story to protect them.

"The hardest for me, the first thing, is mother. The second thing is the life. This is a shit life," he says, in what he calls Jungle-speak English. "If my mother sees I'm in the Jungle, she will be very sad..I don't want to tell her."

Running away

In 2015, almost 90,000 asylum seekers in the European Union were unaccompanied minors under 18, according to Eurostat. That's up about nine times from just three years ago.

About half run away from asylum centers or shelters within two days of their arrival, according to Missing Children Europe, which represents nonprofits in 24 countries. Sometimes they are discouraged by how long it takes to get legalized, or fear being sent home or to the country where they first arrived. And sometimes they join family members, or just try their luck at asylum elsewhere.

European Union officials say they have developed projects to improve the reception of minors, provide them with health assistance and relocate or unite them with relatives faster. Germany, for example, provides minors who stay at shelters with about 30 to 70 euros ($39-$79) a month in pocket money.

"European legislation pays particular attention to the rights and needs of unaccompanied minors seeking asylum," European Commissioner Dimitris Avramopoulos told The Associated Press. "Implementing these rules is therefore our priority."

However, missing young migrants, most of whom are 14 or older, are treated very differently from missing children in general. For example, the EU Fundamental Rights Agency recommends they be assigned a guardian responsible for reporting disappearances to police, but that seldom happens. Even when cases are reported, police seldom have enough information to follow up, such as a photo of the child, personal data or fingerprints.

Suleiman, a 17-year-old from Morocco, ran away from a shelter several times before ending up on the streets of Stockholm in Sweden. Sweden gets by far the most unaccompanied minors applying for asylum — more than 35,000 in 2015. In October, officials in the port town of Trelleborg revealed that some 1,000 unaccompanied refugee children who had arrived over the previous month had gone missing.

Suleiman, puffing on a cigarette, is one of dozens of Moroccan boys in Stockholm, where he and his friends steal food from supermarkets.

As a street kid in Morocco, he heard stories about how other poor people had left for Europe, and specifically Sweden. After several unsuccessful attempts, he managed to sneak aboard a boat leaving for Europe. He passed through several European countries before he got to Sweden.

There, he was sent to a refugee shelter in Kiruna, a town of 20,000 people in the north, where the winters are dark and very cold.

The cold gave Suleiman back problems, he says. Also, used to big cities with many people around him, Suleiman now felt he had nobody. After running away from the shelter several times, he was placed in compulsory psychiatric care, which he seemed to see as punishment.

"I hadn't done anything," he says. "First time I was there for two months, even though I wasn't guilty of any crime or taking drugs. Now I've managed to stay hidden on the streets of Stockholm for over a year."

At risk

The biggest danger for minors on the move is that they will fall into crime, trafficking or illegal labor, which Europol expects to increase rapidly.

"They are vulnerable, no social network, weak links to national authorities, not understanding the legal procedures, sometimes being treated as illegals," says Robert Crepinko, head of the migrant smuggling unit. "You can see all the boxes ticked."

Abib, an Egyptian migrant who arrived in Italy as a minor and is now 18, acknowledges dealing drugs.

"I buy 50 or 100 euros of drugs, and I can earn 200 or even 300 euros a day," he boasts.

Sometimes, he says, people hire a minor to move big orders of drugs inside school backpacks for just 30 to 50 euros. Sex trafficking also happens at the Termini station, he adds, in exchange for clothes or money.

Emanuele Fattori, police chief at the station, says minors are used for drug trafficking because both the penalty and the chance of arrest are lower for them. And recently, he says, a pedophile priest was arrested for paying underage migrants at the station.

The temptation of crime and illegal labor is greater because so many unaccompanied minors are in debt. All 11 Egyptian teens the AP interviewed say they feel an obligation to pay back the substantial sums of money — usually around 3,000 euros — their families paid for their passage to Europe with smugglers.

Kids who work at car washes can put in 12 hours for just two to three euros per hour, according to Save the Children Italy. And those who work at the Guidonia fruit market load and unload up to 12 pallets in two hours for 10 euros. Police have cracked down on illegal labor inside the market, with 10 people arrested and 50 minors found unloading fruit boxes.

"It is very difficult to find minors who report these situations of exploitation to the police, because they are aware that if they report they will never find a job again," says Antonio Di Maggio, a local police officer in Rome. "There is a big conspiracy of silence."

Seventeen-year-old Ahmed woke up at 2 a.m., took two buses to the station and then hopped a train to get to his job at a car wash outside Rome. There he worked 12 hours a day for 30 euros, for 10 days. When he quit, he was only paid for five.

"Of course I have to send money home," he says. "But I don't have a job, so how can I?"

Said, his hair shorn close to the head, has to help pay back the 3,000 euros his family spent on sending him to Italy.

Said worked two days at the seaside west of Rome selling cold drinks to beach-goers from 8 a.m. until 6 p.m. His daily take-home was just 15 euros. When the summer was over, he spent some days — he won't say how many — moving crates of fruit for 5 euros a day, well below the market rate. Both Said and Ahmed stay in a shelter at night.

"As long as I am here in Italy, there is no future," he says, echoing a refrain of many young Egyptian migrants. "I have to go to France."

Life in The Jungle

Yet France is not the haven for young migrants that Said and his friends imagine.

Earlier this year, more than 325 migrant children were living without mother or father in the Jungle, which is widely considered Western Europe's largest makeshift migrant camp. The southern half of the camp was demolished in March, forcing those who remained to resettle in the north. In the process, 129 unaccompanied children went missing, according to a head count by British humanitarian group Help Refugees.

In April the Defender of Human Rights, Jacques Toubon, a state-appointed but independent watchdog, described "the alarming character" of the situation for unaccompanied minors in Calais and suggested French authorities have failed to keep count of them. It is not an easy task in an underground culture where each day is temporary because tomorrow, they hope, things might change. France Terre d'Asile, a major association, says that of 1,400 unaccompanied minors signed up in 2015 at a center in northern France, only 90 stayed.

Living conditions inside the camp have improved but are still "shocking," says Doctors Without Borders regional coordinator Olivier Marteau. "We're in a camp that doesn't have what you would find overseas."

Many of the unaccompanied children here are traumatized. Some are wily, inattentive and unruly, while others are cautious and quiet. There are few smiles.

Shahzeb, a 15-year-old from Afghanistan, holds a distant gaze as he talks, then abruptly cuts off conversation. His older brother was forced to join the Taliban and then killed at the age of 15 or 16, he says. The Taliban came to his house day and night with guns.

So his father sold the family's land to an agent to pay to send Shahzeb to Europe. The journey took about two months.

Shahzeb has now spent about five months in the Calais camp — the past month in Container No. 53, one in a fenced-off field of heated white containers holding up to 1,500 people. Kids on their own were given access only in late February, long after the start of the harsh winter.

Each night, he tries to slip into trucks making the ferry crossing to Britain. During the day, he sleeps.

"The hardest moment is that every night, we are trying to cross the border. Living in the Jungle is too hard, but when I go there (at night) and face the failure, I'm feeling so bad," he says.

He hasn't spoken to his parents in months. He imagines they think he is happy. He doesn't tell them the truth.

Izhar Ali in Container No. 54 tells a similar story. The 16-year-old's father, a school worker in Pakistan, sold the family house to pay $13,000 for him to get to Britain and study. Izhar Ali says he missed up to five months of school a year in Pakistan because of the Taliban threat.

Izhar Ali talks to his father every four to five weeks.

"He says, 'Send pictures," Izhar Ali relates. "This Jungle is bad..I go to England and send a picture."

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Mike Corder in Belgium and Elaf Ali in Sweden contributed to this report.