American thrillers return constantly to federal investigators, and the reason is jurisdictional rather than patriotic. A national agency solves plotting problems that a local police department creates for the writer.
Jurisdiction determines how far a plot can travel
A city detective is bound to one city, and a story that moves to another state has to invent cooperation, permission or an implausible vacation to keep the character involved.
A federal investigator carries authority across state lines by default, which means the villain can move and the protagonist can follow without the writer stopping to explain it.
For a plot that spans a country the size of the United States, that single property removes most of the mechanical obstacles to keeping the story moving.
Agencies supply hierarchy and friction
Large organizations have supervisors, competing offices, headquarters priorities and procedures, all of which can obstruct an investigator who is right about the case.
That gives a writer conflict inside the protagonist's own side, which is more useful than conflict with the antagonist alone because it can be introduced at any point.
Interagency rivalry does the same work at a larger scale, letting two sets of competent people obstruct each other without either being wrong or corrupt.
Resources make ambitious plots plausible
Forensic laboratories, surveillance capability, aircraft and specialist units let a story include capabilities that a small department could not credibly deploy.
Readers accept these resources without explanation, so the writer can move a team across the country overnight and nobody stops to question the budget.
The same resources also justify the scale of the threat, since a case severe enough to warrant them is by definition worth a reader's attention.
The public already knows the shape
Decades of films, television and news coverage mean readers arrive knowing roughly what these agencies do, what their training looks like and how their teams are organized.
A writer can therefore drop a character into a briefing room and skip the establishing work that an invented organization would require.
Shared knowledge also means small deviations register as deliberate, so an author can characterize an agency by how their version differs from the expected one.
Authority is what the genre keeps testing
Because these institutions hold real power, thrillers use them to ask whether that power is being exercised properly, and the answer varies enormously between books.
Some novels treat the agency as competent and the individual as the problem, while others reverse it entirely and make the institution the antagonist.
The setting persists across both because it is the most convenient place in American fiction to stage an argument about official power.