In personal injury cases in the U.S.A., large amounts of money can be involved and lawyers work for contingent fees. Thus it can be worth a lawyer's while to pay a substantial bribe to a judge.
Scott Turow's novel Personal Injuries is about an FBI investigation of judges in the fictitious Kindle County who take bribes. (All of Scott Turow's novels are stories about lawyers and judges in or from Kindle County in a fictitious midwestern state whose name is never mentioned.)
These are not federal judges. So why would it be the FBI rather than authorities of the state in which Kindle County is located?