I looked this up before posting my comment. According to Wikipedia at https://en.wikipedia.org/wiki/Witness_immunity https://en.wikipedia.org/wiki/Witness_immu

I looked this up before posting my comment. According to Wikipedia at https://en.wikipedia.org/wiki/Witness_immunity https://en.wikipedia.org/wiki/Witness_immunity: "In the United States, Congress can also grant criminal immunity (at the Federal level) to witnesses in exchange for testifying.[1]"