The use immunity statute (18 U.S.C. § 6002) allows the government to prosecute the witness using evidence obtained independently of the witness's immunized testimony. Section 6002 provides:
[N]o testimony or other information compelled under the order (or any information directly or indirectly derived from such testimony or other information) may be used against the witness in any criminal case, except a prosecution for perjury, giving a false statement, or otherwise failing to comply with the order.
The Supreme Court upheld the statute in Kastigar v. United States, 406 U.S. 441 (1972). In so doing, the Court underscored the prohibition against the government's derivative use of immunized testimony in a prosecution of the witness. The Court reaffirmed the burden of proof that, under Murphy v. Waterfront Commission, 378 U.S. 52 (1964), must be borne by the government to establish that its evidence is based on independent, legitimate sources:
This burden of proof, which we affirm as appropriate, is not limited to a negation of taint; rather, it imposes on the prosecution the affirmative duty to prove that the evidence it proposes to use is derived from a legitimate source wholly independent of the compelled testimony.
Kastigar, supra, at 460.