Day 4: 2:00 P.M.-3:00 P.M. - S4-E8
Factual error: When Edgar proves that Mary-Ann Taylor framed Sarah to be the rat in CTU, he claims that she remotely took control of her computer by sending the signal over "a plain old AC power line." This is not possible, unless the computers were equipped with special hardware to separate the AC from the signal. If they were, this traffic would have been observed by James Heller's assistant.
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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