Attorney General William Barr has reported to Congress that special counsel Robert Mueller has cleared President Trump and his campaign team of claims of conspiring with Russia during the 2016 election. This is more than an exoneration. It’s a searing indictment of the Federal Bureau of Investigation, as well as a reminder of the need to know the story behind the bureau’s corrosive investigation.

Mr. Mueller’s report likely doesn’t put it that way, but it’s the logical conclusion of his no-collusion finding. The FBI unleashed...