Day 4: 9:00 P.M.-10:00 P.M. - S4-E15
Continuity mistake: Chloe tells Michelle that she's confirmed Marwan's identity using voice tags from McLennan Forrester, but all of the computer records at MF were wiped out a few episodes earlier by the EMP, according to both the MF employees and CTU. The fact that they didn't get any information from the company except for the printouts Jack and Paul extracted was a major plot point a few hours ago, so even if a real EMP wouldn't do this, the fact that it's stated repeatedly in the show means it should hold true within the series; Chloe suddenly having information from them an hour or so later is discontinuous, and thus a mistake.
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.
Cubs Fan ★