FRONTLINE and ProPublica go inside Europe’s fight against terrorism — the missed warnings and the lingering vulnerabilities.

Thirty years after a forgotten massacre that occurred during the Guatemalan civil war, a forensic scientist and prosecutor search for Oscar, a young boy who survived the horror.

FRONTLINE investigates American-born terrorist David Coleman Headley, who helped plan the deadly 2008 siege on Mumbai. In collaboration with ProPublica, the film — an updated and expanded version of A Perfect Terrorist — reveals how secret electronic surveillance missed catching the Mumbai plotters, and how Headley planned another Charlie Hebdo-like assault against a Danish newspaper.