propublireporter sebastian rotella has been covering terrorism for two decades. years before the attacks, he was already reporting on some of the jihadists who would go on to strike europe, and the counterterror officials trying to stop them. in this film, he sits down with the men and women on the inside of the fight against al qaeda and isis. they reveal the missteps and systemic breakdowns that allowed known terrorists to hit the heart of europe, how the problems persist today, and the unprecedented threat the continent faces. >> (translated): it's a disaster. why were they not monitored and stopped? these people were time bombs. >> these individuals were on the radar, they had traveled to syria, they were known to law enforcement intelligence officials. >> no system is perfect. and we live in a free world. how much of your freedom do you want to sacrifice for your security? (gunfire, explosion) >> (translated): the november 13th attacks should never have happened. the brusslls attacks should never have happened. the system has completely failed. >> narrator:

. >> frontline and propublica reporter sebastian rotella investigate the failures. >> rotella: how can known terrorists travel through europe so easily? >> and why europe remains so vulnerable. >> there's every reason to expect that we'll see isis lash out while it's under pressure to maintain its relevance. >> "terror in europe." >> go to pbs.org/frontline for more on what nations in the middle east are doing to combat isis, and how that's impacting the terror group. read extended interviews with chuck hagel, >> i didn't think i overstated it; i think i said it exactly right. ashton carter and others. >> we can't fuel sectarianism. >> connect to thfrontline community on facebook and twitter. then sign up for our newsletter at pbs.org/frontline. >> frontlinis made possible by contributions to your pbs station from viewers like you. thank you. and by the corporation for public broadcasting. major support for frontliis provided by the john d. and catherine t. macarthur foundation, committed to building a more just, verdant and peaceful world. more information is available at macfound.org