JUDY WOODRUFF:

Now we continue our series Inside Putin's Russia, and travel to the country's southernmost border.

The Republic of Dagestan is in the North Caucasus, near the Caspian Sea. Over the past two decades, a brutal separatist insurgency has fought the Russian state, and violence has spilled over from neighboring Chechnya, where Russia fought two wars.

Many Americans may recall Dagestan as the original home of the Boston Marathon bombers, but now a new problem. By one estimate, as many as 5,000 Dagestanis are fighting for ISIS.

Again, in partnership with the Pulitzer Center on Crisis Reporting, special correspondent Nick Schifrin and producer Zach Fannin examine why.

KAZIM NURMAGOMETOV, Father of ISIS Fighter: It is no accident that the youth are tempted to go to Syria, because, today, there is a revival of Islam.