The FBI tells the Military What to Do?

[KarlGauss]

In this link describing the accounts of Saddam’s US military guards, it states:

(the guards were from) C Company, 2nd Battalion, 103rd Armor Regiment, a Pennsylvania National Guard unit from the Scranton area that was activated for duty in Iraq in late 2003. Instead of combat, they were chosen by the FBI to serve as guards at a U.S. military compound where Saddam was

Is this typical? Do outside agencies actually have the power to the Army what to do, which unit to choose?