In the wake of 9/11, a series of letters laced with a deadly powder called anthrax appeared on the desks of prominent journalists and politicians in New York City and Washington D.C. Five people were killed, and seventeen more were infected. It was the worst case of bioterrorism in American history, setting off a surge of finger pointing, xenophobia, and fear. But what started as an unprecedented case soon turned into an unsettling mystery. Who had sent these letters? And why?

On Season 4 of Cover Up, we trace the FBI’s costly investigation into the attacks, which was stacked with a seemingly endless series of false leads, damaging leaks, and embarrassing blunders. Through revealing interviews with victims, agents, and suspects, we unpack the case in its messy entirety, all with an eye toward the question that remains today: Did the FBI identify the true killer, or is this case still unresolved?