Robert Mueller's investigation: Understanding an expert at work

Like a fishing trawler on the horizon, the real action involving special counsel Robert Mueller's investigation into the Trump administration is beneath the surface. As a former federal prosecutor who now teaches criminal law, I find myself answering a lot of questions about what is likely happening. They are good questions, and important.

The latest public development — and the most important one so far — was Friday's bombshell announcement that former national security adviser Michael Flynn was pleading guilty to a felony and working with the special counsel's office in its pursuit of other targets. In the parlance of my field, Flynn had "flipped." This is important, because Flynn worked for Trump before, during and after his transition to office and had a high-level role throughout. Moreover, he admitted in his statement of the offense (a document that accompanies a plea agreement) that he received direction from a "very senior member of the Presidential Transition Team." He has almost certainly been a mother lode of information for Mueller.

So how did this deal get made? For many people, their understanding of this crucial step in a complex criminal investigation is defined by what they have seen in movies and television dramas. On a show like "Law and Order," witness/defendants often flip, and it is a simple process: The defense attorney makes an offer for a specific charge or sentence (for example, "manslaughter one" or "three years") in exchange for cooperation, and the prosecutors accept. Then the new cooperator begins to spill out information. Crying often ensues.

The reality is almost always more complicated.

The first step on "Law and Order" is realistic: The defense might approach the prosecution to suggest an openness to cooperation. Particularly where harsh guidelines or mandatory minimum sentences are in play, this can be an alluring avenue for someone targeted by an investigation, like Flynn. The next step, however, diverges from the TV script.

Instead of cutting a deal before information is divulged, a prosecutor will almost always insist on scheduling a "proffer of information," where the target and his attorney will meet with the prosecutor and investigating agent so that the target can set out what he knows.

Before actually sitting down for that meeting, though, the attorneys will make clear what the terms are for the meeting itself; there are rules for these things, after all. Almost always, those terms will be set out in a "Kastigar letter" (named for a Supreme Court case) that the prosecutor will present to the defense attorney. The Kastigar letter will give the target "limited use immunity," meaning that the government cannot use what the target says directly against him in a later proceeding. In other words, the feds couldn't take a proffer from Flynn, then later use what he said at the proffer to convict him in a later trial. There is a twist, though: The government can still use this material to impeach the target if he says something different under oath down the road. Effectively, that means that once someone has proffered information, they very rarely would want to testify on their own behalf at a later trial. They are, in the lingo of the trade, "locked in."

Now it is time for the proffer session. The purpose of the proffer is to allow the government to hear and assess the information the target can offer. They listen not only to what the witness/defendant says, but how he says it. The proffer will start with "control questions" — questions that the government already knows the answers to through other evidence.

The target, of course, probably does not know what information the government has, particularly when the target (like Flynn) has not been charged and has no right to discovery of the government's evidence. The use of control questions means that any decent defense attorney will strongly advise a client that if they are going to give a proffer, they must do so honestly.

The bare mechanics of this process give the government three great advantages in the deal making that follows. First, prosecutors get information from the target before the target gets a deal. Second, once a target has given a proffer, he is locked into a story. Finally, control questions can guarantee that information given by a target is probably worthwhile.

Only after all of this is completed will the defendant get a plea offer, and even that will be contingent on the target successfully performing whatever tasks the government might require. Flynn's plea agreement, for example, sets out that his cooperation may include "answering questions; providing sworn written statements; taking government-administered polygraph examination(s); and participating in covert law enforcement activities," and testifying in grand jury and other proceedings. If he refuses or is dishonest, the government can charge him with additional crimes, but he cannot pull out of the plea of guilty he has already made to the existing charge.

When multiple targets are proffering information and other evidence is coming in through search warrants and subpoenas, the lead prosecutor has a lot of orchestrating to do. And, by all accounts, Mueller is a maestro. His team is lean (under 20 lawyers, compared with the staff of 225 that Ken Starr used to pursue Bill Clinton) and stocked with remarkable lawyers. One of his hires, for example, was Michael Dreeben, who is regarded by many (including me) as the best federal criminal lawyer in the United States.

While Mueller thinks three moves ahead, it is difficult to tell what the Trump team is doing, other than assuring the president that the investigation will be wrapped up soon. Of course, they might be right about that. And if they are, that might be a bad thing for those close to the president and perhaps for Trump himself.

Mark Osler is the Robert and Marion Short Distinguished Chair in Law at the University of St. Thomas.