It was one year ago that ISIS attacked multiple targets in Paris, killing 130 people. Today, France remains in a state of emergency that was sparked by the attacks, and up to 15,000 people in the country are being monitored for suspected ties to terrorism.

But how do you detect a potential terrorist before they carry out an attack — and at what cost?

FRONTLINE’s « To Catch a Terrorist» follows the case of Karim Hadjidj, a French citizen who spent 15 days in Syria before returning home. Karim is one of the roughly 2,000 people who have left France for Syria or Iraq, forcing police to sift out who among those returning poses a threat.

«To Catch a Terrorist» illustrates how France is dealing with the threat from within, highlighting the challenges authorities face in deciding who to monitor and how. Find it here.