Day 4: 1:00 A.M.-2:00 A.M. - S4-E19
Plot hole: At 01:50 am, Chloe is told to go to Sabir's house. She arrives there approximately 5 minutes later, at 01:55:45. At 01:58, Jack is told that the closest tactical team is 15 minutes away from Sabir's residence. A few moments later, Jack mentions that the tactical team is 10 minutes apart from Chloe, meaning they bothered to send the team over there even though any officer currently at CTU HQ, including Jack, could get over there a lot sooner.
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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