With the news that former National Security Advisor Michael Flynn is seeking immunity for his testimony, it is perhaps worth a few moments reviewing the concept of immunity.
Let's begin with some basics. The idea of "immunity" can have at least three distinct flavors or meanings when used in the context of a criminal prosecution or a Congressional investigation. First, it might be thought of as "transactional immunity." That is, the immunity in question applies to an entire course of conduct or transaction. In the case of General Flynn this would (if it were offered, which it will not be) take the form of a letter agreement from the US government saying, in effect, "if you testify truthfully you will not be prosecuted for anything having to do with Russia." [The latter clause is, of course, important to be written carefully since it delimits the immunity—we won't of course want to excuse an earlier murder and we might not want to excuse, say, the work he did for Turkey.]
The second form of immunity to consider is what is known as "use immunity"—that is the promise (consistent with the Fifth Amendment) that what Flynn testifies to will not be used against him. In other words if he testifies to Congress "I took money from the Russians" then in a later criminal prosecution the government will not be permitted to introduce his sworn testimony from Congress to the criminal jury and say "see ... he confessed."
The third form of immunity to consider is known as "use and derivative use immunity." It's the same as "use" immunity but broader in the sense that the government not only promises not to use the testimony in a later trial but also not to use anything derived therefrom -- i.e. stuff that they learned about because of the testimony in question. So, to follow up on our fictitious example, if Flynn testified "I took the money" from the Russians under use immunity only that testimony would be excluded from a later criminal trial. But we could, for example, use all of the bank account information that showed the transaction to get to the same place. Under use and derivative use immunity, if the US government had learned of the money transaction only because of Flynn's testimony then the bank transaction records would be derived from the testimony and they would also be excludable at trial.
So ... with that background, let's step back and ask another question -- why do we have the concept of immunity at all? The answer lies in the absolute nature of the Fifth Amendment protections. The Fifth Amendment says that a person may not be compelled to give testimony against himself. The courts have interpreted that to mean that a witness may remain silent whenever he has a reasonable belief that his truthful testimony will provide evidence of his criminality.
But there are sometimes the US government may think that the information and testimony is more valuable than the prosecution of the individual in question. Mafia soldiers often hid behind the Fifth Amendment protections to lawfully avoid giving testimony about their bosses (or, more realistically, to avoid being jailed for contempt for refusing to answer questions about their bosses). And so Congress enacted a provision that allowed the government to compel a witness to testify—notwithstanding his Fifth Amendment privilege against self-incrimination—but on the condition that the government also promise not to use that testimony against the witness.
[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). Along the way the Court made clear then (and it remains so today) that simply "use" immunity would be insufficient to substitute for a witnesses' Fifth Amendment privilege. At a minimum, use and derivative use immunity (consistent with the statutory text) had to be provided. Of course full transactional immunity would also suffice since it removes any possibility of subsequent prosecution.
Thus, in Kastigar, 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.
In high profile Congressional investigations, this burden of proving an independent source for evidence can be quite high. Perhaps most famously, Admiral John Poindexter's conviction during the Iran-Contra affair was overturned on appeal precisely because the government could not establish to the satisfaction of the court that its evidence of Poindexter's guilt was independent of his widely publicized testimony to Congress. Indeed, some have suggested that, in effect, for widely publicized Congressional investigations the choice is "either/or"—either have the testimony in Congress or the criminal prosecution. You can't have both. (Certainly you can try by, for example, isolating the prosecution team from all public reports about the testimony—but even that may not be enough. The Poindexter conviction was overturned because witnesses against him had been exposed to his testimony.)
So .. where does that leave us and where does it leave General Flynn?
First, and most obviously, Flynn will likely be seeking as broad a transactional immunity as he can get. He will want it for his interactions with the Russians and the Turks as well as any derivative crimes from alleged failures to report those contacts as required by law.
The Congress, by contrast, should be reluctant to grant immunity of any sort unless it knows what testimony it might be getting. Nobody wants to buy a "pig in the poke." So they will insist on some form of "proffer" from Flynn (an off-the-record summary of his anticipated testimony) on which to base a decision to grant immunity. If the proffer is enticing enough then the Congressional committees will vote to grant immunity (they have their own separate statutory provision permitting this) and, in effect, place an obstacle in the way of any criminal prosecution. How big an obstacle remains to be seen, but the Poindexter case suggests it would be relatively significantt.
For that reason, one final piece of the puzzle might involve the Department of Justice—which, if it has an ongoing investigation, may request that Congress hold off in granting immunity. That has been the case in most instances since Poindexter. And, as a final note, there is at least two other ways to get Flynn's testimony—either as part of a plea bargain in exchange for a lighter charge or sentence OR after a full prosecution and conviction. Those, too, might be on the table for consideration -- but it is something that only DOJ can achieve and typically takes longer than we might like.
I see that as I was writing this co-blogger Andrew Kent put up some similar thoughts that are also worth reading.