Quantico is a town in Prince William County, Virginia, that is home to the FBI Academy. As such, it is often used metonymically to refer to the FBI instead of the city.
During his military service, Frank Castle trained at Marine Corps Base in Quantico. Castle was scout sniper and was graduated of class 307.
As part of FBI National Academy, Dinah Madani received field training in Quantico, where she drew top marks in intelligence analysis, profiling and investigations.
Recruited to the FBI, Benjamin Poindexter was sent to Quantico where he trained in various specifics, including counterintel.