Day 3: 10:00 A.M.-11:00 A.M. - S3-E22
Continuity mistake: During the hostage exchange, there is a shot of the black man waiting for Jane Saunders to walk to him, in the background we can just see a man stood behind a car door. However, in the wider shots and when Saunders jumps out of his car, the man is not there.
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.
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