DailyKos links to a WaPo article:
A grand jury, impaneled in Washington in May, has obtained evidence gathered over at least four months through wiretaps of Washington area cellphone numbers and has scrutinized whether Weldon received anything of value, according to the sources. They spoke on the condition of anonymity because of the sensitive nature of the investigation.
Well, I guess we know where and how that "new evidence" had come from, and that we can infer why the FBI jumped so quickly.