How about this: someone in the agency (either NSA or FBI, you can choose) had it in for Brendan and actively tried to sabotage cases, reports, etc. in an attempt to get him fired. He/she did an excellent job of covering his/her tracks, but was eventually caught and fired him/herself. Things have been quiet for months, but now the jealous/unstable former agent is back and bent on revenge. I'd like to leave the how/why/where up to you, but if you'd rather have suggestions I'd be happy to provide a few. :)