The issue I have with this is that there's no such thing as "by the book" Federal investigations anymore. Snowden showed us that US intelligence agencies are using illicit, warrantless means of obtaining information and then providing that information to law enforcement agencies. Those agencies then go to judges and request warrants, and they can prove that the warrant is justified because they already have the damning evidence that they're legally supposed to require the warrant to get. Which renders them meaningless on a federal level.