PARIS -- The teenage sisters told their father they were staying home sick from their suburban Denver school. Instead, they took $2,000 and their passports and headed off for Syria with a 16-year-old friend. They made it as far as Germany before border guards detained them for questioning.

The fact that adolescent girls could make their way across the Atlantic might come as a surprise to many parents, but a patchwork of laws and rules governing international air travel in many cases makes it easy for teenagers to travel with nobody's permission but their own.

Airlines have a range of rules governing minors' travel: Many major carriers including United Airlines and Scandinavian airline SAS place no restrictions on children over 12, while others let even young minors travel as long as they are accompanied by someone over 16. Yet others, including American Airlines, require a parent to accompany travellers under the age of 15 to the gate, while those 15 and over face no restrictions.

Countries have a separate set of laws that is no less haphazard, from a Russian requirement for notarized parental permission to the U.S. system where adolescents with valid passports are free to come and go.

In Spain, both parents must fill out a permission form at a police station before a minor can travel alone. In Germany, where the American teens were stopped, border guards are required to verify that minors have parental permission to travel.

And in France, which is Europe's single largest source of would-be jihadis, parental authorization had to be received by city hall -- until January 2013.

That's when a small administrative change took effect suspending the requirement for parental approval. The government said it would streamline unnecessary bureaucracy and officials remarked that few runaways went abroad, and even fewer stayed there.

Fast-forward 22 months, and nearly every week new reports emerge of French adolescents leaving for Syria. Teenagers from France can travel within the European Union with a valid ID; outside the EU, they need only a passport.

Under French law, parents can have their children flagged if they fear they will leave the country to join extremists. But for many of those who have left, their families had no warning.

Lawyer Agnes Dufetel-Cordier represents a teenage boy from Toulouse who left in January to join the al-Qaida-linked al-Nusra Front, before coming back to his family to face criminal charges. She said the teen -- now 16 -- gave no sign he was about to bolt for Syria, and his departure came as a shock to his parents. He was not stopped at the airport in Marseille, nor on arrival in Turkey or crossing into Syria.

"If you reverse the regulation that lets them travel without their parents' permission, you will see right away that minors are no longer leaving," Dufetel-Cordier said. "Today in France, in the case of most minors who go to Syria, the families have absolutely no idea ahead of time."

At age 17, Sahra Ali Mehenni went so far as to ask her mother to get her a passport, saying she wanted her paperwork in order before she reached adulthood. When she left for Syria on March 11, departing from the Marseille airport just as the teen boy did, she took her burgundy-bound passport and nobody stopped her before she boarded the flight to Istanbul.

"If I go out and I run a red light, they're going to get me right away," said her father, Kamel. "But these minors are going to Istanbul -- and if they're going to Istanbul, it's to go to Syria. They know it. You can't say they don't know it. And no one stops it."

The same concerns apply in the United States, where parental permission is required to obtain a passport -- but none is needed for travel.

The teens who flew to Germany were stopped because their parents, part of suburban Denver's tight-knit East African community, reported them missing quickly, and American authorities contacted German authorities before they landed.

"Kudos to those family members in Colorado," said Omar Jamal, CEO of American Friends of Somalia. "Time is of the essence. They did the right thing."

Associated Press writers contributing to this story included Amy Forliti in Minneapolis; Ellliot Spagat in San Diego; Geir Moulson in Berlin; Vladimir Isachenkov in Moscow; and Lara Jakes in Washington.