If the FBI sits back, they become complicit in whatever illegal activities were happening. Then you can have situations like the ATF gunwalking scandal (http://en.wikipedia.org/wiki/ATF_gunwalking_scandal) in which the ATF was trying to track smuggled guns up to higher-level suppliers, and had one of those guns used in a murder.
This would presumably go so far as the FBI asking other agencies to cease investigations which might compromise their larger operation. At some point, they will have to act.
If the FBI sits back, they become complicit in whatever illegal activities were happening. Then you can have situations like the ATF gunwalking scandal (http://en.wikipedia.org/wiki/ATF_gunwalking_scandal) in which the ATF was trying to track smuggled guns up to higher-level suppliers, and had one of those guns used in a murder.
This would presumably go so far as the FBI asking other agencies to cease investigations which might compromise their larger operation. At some point, they will have to act.