The FBI Truck is an Armoured Personnel Carrier (APC), similar to the SWAT Tank. It is a rugged near-indestructible vehicle and will not often flip on rough terrain.
As with the Andromada, it is never seen on any mission or on the streets, and it is not used by the FBI to chase you when you have a Wanted Level. The only way to obtain it is by a third party trainer program or garage editor.
Rockstar also tried to put an inbuilt minigun on the FBI Truck, but didn't done this.[sup](?)[/sup]