Posobiec: "Name one mass shooting that the FBI has stopped." (video)
Jack Posobiec makes a good point here. There have been several mass shooters who were either known to the FBI or in actual contact with FBI informants/agents shortly before the murders.
One might say that we only know about the FBI's failures. Sorry, criminal events are not classified and the FBI since Clinton was President has been desperate for good press. Successes would not just be leaked, they would be publicized.
What the FBI has been caught doing is manufacturing evidence, entrapment, lying to the FISA courts, lying to judges, illegally withholding discovery and more.