Brown v. Walker
In Brown v. Walker, 161 U.S. 591 (1896), it was urged that the prohibition against prosecution did not grant protection against prosecution in the state courts.
First finding that Congress could constitutionally provide such immunity, the Court then interpreted the statute:
"The act in question contains no suggestion that it is to be applied only to the Federal courts. It declares broadly that `no person shall be excused from attending and testifying . . . before the Interstate Commerce Commission . . . on the ground . . . that the testimony . . . required of him may tend to criminate him,' etc. `But no person shall be prosecuted or subjected to any penalty or forfeiture for or on account of any transaction, matter or thing concerning which he may testify,' etc. It is not that he shall not be prosecuted for or on account of any crime concerning which he may testify, which might possibly be urged to apply only to crimes under the Federal law and not to crimes, such as the passing of counterfeit money, etc., which are also cognizable under state laws; but the immunity extends to any transaction, matter or thing concerning which he may testify, which clearly indicates that the immunity is intended to be general, and to be applicable whenever and in whatever court such prosecution may be had." 161 U.S., at 607-608.