Under whose custody?

Assistant Director Marsh:

[Carl and Mr. Marsh are visiting Frank in prison; Carl hopes to convince the FBI to let Frank out of prison]
I'd like for you to take a look at something, tell me what you think. Frank Abagnale, Jr.:
[Marsh hands Frank a fake check as Carl looks on]
It's a fake. Assistant Director Marsh:
How do you know? You haven't looked at it. Frank Abagnale, Jr.:
There's no perforated edge, right? This check was hand-cut, not fed. The paper's double-bonded, much too heavy to be a bank check. Magnetic ink, it's raised against my fingers, not flat. This doesn't smell like MICR, it's some kind of, uh, some kind of drafting ink. The kind you get at a stationery store. Assistant Director Marsh:
Frank, would you be interested in working for the FBI's Financial Crimes Unit? Frank Abagnale, Jr.:
I've already got a job here, you know. I, uh, deliver the mail. Assistant Director Marsh:
Frank, we have the power to take you out of prison. You'd be placed in the custody of the FBI where you'd serve out the remainder of your sentence as an employee of the Federal Government. Frank Abagnale, Jr.:
Under whose custody? [Carl raises his hand]