<Li> Jeff Daniels as John O'Neill, the chief of the New York FBI's Counterterrorism Center known as "I - 49". He is convinced that the United States has been targeted for attack by Al Qaeda but is faced with hostility from other agencies of the federal government, particularly from the CIA . </Li> <Li> Tahar Rahim as Ali Soufan, a Muslim Lebanese - American FBI agent on John O'Neill's I - 49 counterterrorism squad who eventually becomes his protégé . He is infuriated by the perversion of Islam by enemies of the United States and goes so far as to go undercover in terrorists' gathering spaces, in order to hunt Al Qaeda and prevent attacks . </Li> <Li> Wrenn Schmidt as Diane Marsh, a CIA analyst who works under Martin Schmidt . Similar to Schmidt, she believes that the CIA is uniquely positioned to combat terrorist attacks and therefore decides to conceal information from the FBI . The character is based on at least three people within the CIA including Alfreda Frances Bikowsky . </Li> <Li> Bill Camp as Robert Chesney, an FBI veteran in New York's counterterrorism unit . About to retire, he uses his skills at interrogation to extract important intelligence from suspects in the struggle against terrorist threats . Chesney has been described as the most composited of all the main characters with Soufan commenting that he is "at least four people ." </Li>

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