We really don't know when the investigation started on all of this, do we? We know Berger's first documents theft occurred in October, but from what I understand, Bruce Lindsey was contacted and the documents were returned. We've been told that he pilfered documents on more than just this occasion, and that his home and office were searched in February (I believe). This leads me to believe that the criminal part of the investigation didn't begin until the searches were conducted. Is there a specific date given as to when he became a Kerry adviser? Since he wasn't on the payroll, he could have been advising Kerry from the start.
I think I read the FBI got involved (or as Lanny Breuer, Berger's attorney says "3 months later decided to get involved", as if inserting themselves inappropriately) in January. That's why we're hearing the phrase "3 months later" referring to October until January.
Oh--and the Republicans were concerned about a press briefing Berger gave *on Kerry's behalf* on February 27, to help your timeline.