At the tender age of 16, Ahmad Fawad has three fears: He fears the Taliban gunmen who killed his family members in Afghanistan. He fears that once-welcoming European countries are shutting the doors after the carnage in Paris. And he fears that the very extremists who attacked his family at home are moving alongside him in the flow of humanity bound for Europe.

As four Balkan nations launched partial border closures Thursday, Fawad's concerns were echoed along the trail that has brought hundreds of thousands of people fleeing war and poverty into Europe this year. Migrants interviewed by The Associated Press voiced sympathy for the 129 victims of the Paris attacks along with hope that European nations won't let fear of extremists change their own societies.

They also joined calls for Europe to make a better effort to weed out potential extremists within the refugee ranks, so that the door is left open only for those in need of help.

But already Thursday, signs emerged that countries in Europe are rethinking their refugee policies. Serbia, Croatia, Slovenia and Macedonia shut their borders to people coming from countries not at war — such as Sri Lanka, Pakistan and Iran — and left them open only to refugees from war-torn countries, primarily Syria, Iraq and Afghanistan.

The overnight decision may reflect the beginning of a broader anti-migrant backlash following evidence that emerged from the Paris attacks that militants may have come in among the flow of refugees.

Thursday's moves could trigger huge pileups of desperate people along the Balkan corridor that has become the main route to wealthy EU states, mostly Germany. Although Syrians fleeing civil war are the biggest group among the asylum-seekers, tens of thousands of people escaping poverty have also joined the surge.

"We're trapped," said Mohammed Mirzam, an Afghan who could not cross from Greece into Macedonia because his wife and children are Iranian. "They won't let my family across. We have no money, and we're waiting without any idea of what is to happen. We want to reach Germany or Switzerland."

Migrants such as Fawad said they were leaving their own countries to avoid just the kind of violence that happened in Paris. The teenager fled his predominantly Shia village after gunmen from the Sunni fundamentalist Taliban killed his father and a younger brother. He reluctantly left behind his mother and sister, because the family couldn't afford to pay smugglers for all three.

He had planned to try to make it to Sweden, where he has relatives. But he abandoned his dream after Sweden reinstated border controls last week, and now is aiming for Switzerland. He said the Paris slaughter was certain to sour Europe's willingness to shelter people like him.

"They (Europe) will be more strict for refugees," he said, adding wistfully: "My relatives say they are so happy (in Sweden). That is why I decided to leave my country."

For Ferhad Nezdevan, a 29-year-old Syrian, the Paris attacks cast a cloud of suspicion over all those seeking refuge in Europe.

"It's a problem for us," Nezdevan said on the Slovenian-Austrian border. "In Syria there is war. Here there is war."

Nezdan left his home because of violence against his family by the Islamic State group, which is battling the Syrian government and controls large swaths of Syria and Iraq.

Mistrust of refugees streaming into Europe has grown after a Syrian passport, registered in countries along the migrant route, was found next to the body of one of the Paris attackers — even as officials appealed against hasty conclusions.

European security officials and the Frontex border agency have consistently said they see no evidence of extremists traveling among refugees. Militants, they argue, are more likely to arrive through Europe's airports with good paperwork than take the risk of a difficult and dangerous journey by sea and overland.

It now seems clear, however, that the attacker with the passport did travel the migrant route. Authorities in Greece, Serbia and Croatia say a migrant presented the passport while traveling through their nations last month, and that the man's fingerprints match those of the dead bomber.

Now the key question is whether the passport ended up next to the attacker by chance, or was part of a deliberate plan to sow fear about migrants and generate a European backlash. German Interior Minister Thomas de Maiziere said the passport most likely represented "a trail that was intentionally laid."

Officials also contend that — given the tens of thousands streaming by the month into Europe — it is impossible to keep out a few dangerous individuals. Police check arrivals against Interpol lists of people with criminal convictions or inclusion on intelligence agencies' terror watch lists. But checks are often gravely hampered by migrants' lack of verified ID, leaving police to record whatever name, address and other personal details are provided, along with their fingerprints.

Some on the migrant trail also suspect militants are in their midst.

Fawad said that along his trek through Iran, Turkey, Greece and the Balkans, he has encountered Taliban activists and overheard blood-curdling talk from Pakistani travelers. He spoke of three men and a woman who declared they wanted to join IS. Why, he wondered, would they then seek shelter in the West?

"Some Muslims are not Muslims," he said. "They don't act like Muslims. They are infidels, total infidels. They do crimes in the name of Islam."

Others say European nations should step up security checks so that militants don't sneak in with the refugees and leave them exposed to precisely the kind of violence they are trying to escape.

"Europe made a big mistake. They should not allow in all of the people," said Emile Tarabeh, a Syrian customs officer who passing through security screening near Serbia's southern border with Macedonia.

As Serb immigration officers ran newly arrived foreigners through a metal detector and confiscated cosmetic scissors and cooking knives, they appeared determined to let the human tide reach the next country without thorough scrutiny.

Virtually all migrants passing through Serbia hope to reach Germany or other rich western European nations. At the borders, the security focus has been on identifying human traffickers, not possible terrorists.

Mohamad Nouv Khanji, a Syrian doctor, observed the huddled masses packed around him in a Serbian holding pen for migrants.

"I am sure there are a lot of terrorists," he said. Europe should come up with "a good strategy to know that this is a good man, or this is not a good man."

Khanji traveled with his passport, but Frontex says most migrants enter Europe with no valid documents. In a growing number of cases, they carry fake IDs as they pretend to be Syrian to improve their asylum chances.

That also has raised fears of Islamic extremists entering Europe with false documents.

And the migrants now fear that IS may transport Syria's civil war to Europe — targeting rival Syrian groups as well as European society.

"They did the same in Paris like they do in Syria," said Tarabeh, the customs officer, gently rocking his 4-month-old boy. "I could have stayed in Syria."

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Shawn Pogatchnik in Dublin and Lorne Cook in Brussels contributed to this report.