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In Florida, a man mysteriously falls ill and dies. Special Agent Matthew Ryker suspects an anthrax attack - how many others have been infected?

Ryker’s investigation moves to New York as another case of anthrax occurs. Bruce Ivins begins having suspicions of his coworkers.

An attack on Capitol Hill sends Ryker and his team to a mail facility that’s become a hot spot for anthrax infections.

The FBI learns more about Bruce Ivins’ peculiar past behaviors after an old colleague raises red flags. Ryker and Dani trace the origin of the letters.

All eyes shift to “Mr. Z.” As they start to zero in on a suspect, Ryker and Dani get new assignments - but that won’t stop them from chasing their lead.

With two suspects in question, Ryker’s own beliefs about the case are tested.