It refers to law enforcement, period. The Fifth Amendment doesn't really care what kind of badge that you're being interrogated by, and that's what this case was addressing, since it was a SCOTUS case.
This particular case involved the IRS, but the IRS are federal agents, as are FBI. It explains this in the summary, if you don't want to read the full ruling.
Is that referring to the FBI?
It refers to law enforcement, period. The Fifth Amendment doesn't really care what kind of badge that you're being interrogated by, and that's what this case was addressing, since it was a SCOTUS case.
This particular case involved the IRS, but the IRS are federal agents, as are FBI. It explains this in the summary, if you don't want to read the full ruling.