That's less clear than it might seem; the information Patterson gave them may have been sufficient basis for probable cause against Shafer, but it was probably shaded (at least by omission) in a way that it did not do so against Patterson.

Now, obviously, one would hope that the FBI would do some meaningful additional investigation before conducting a raid, but there were very few people beside the person they'd been handed as a subject who would have been able to provide information which would have flipped this to something where Patterson would be the offending party (and even there, its for something which the FBI is neither the usual first investigating agency nor an agency that is particularly expert.)