At the heart of special counsel Robert Mueller’s prosecution of former Trump campaign chair Paul Manafort, there are several somewhat mysterious — and related — questions.

What is Mueller trying to achieve? Does Manafort have the goods on collusion? And why isn’t Manafort flipping?

All these have been the subject of rampant speculation since Manafort was first indicted by the special counsel’s team last October — and yet the answers remain elusive. (Manafort is now facing 25 charges, has pleaded not guilty to them all, and was hauled off to jail to await trial on Friday.)

Mueller’s investigators have been interviewing a plethora of people on many different subjects, and he’s gotten plea deals from three other former Trump aides. But publicly, more of the special counsel’s activity has been focused on Manafort than on any other single person involved in the Russia investigation.

That’s for good reason. Manafort’s ties to Russian interests are longstanding. He had spent years working for pro-Russian politicians in Ukraine. He was heavily indebted to a Russian oligarch and secretly contacted him during the campaign. A longtime business associate of his, whom he was also in touch with during the campaign, is allegedly tied to Russian intelligence. And Manafort was involved in suspicious events such as Don Jr.’s Trump Tower meeting with a Russian delegation too.

So if you’re investigating whether the Trump campaign colluded with Russia, you’ll naturally investigate Paul Manafort.

And yet none of those 25 charges against Manafort are explicitly about that central question of collusion to interfere with the presidential campaign. Instead, they’re about his older Ukrainian lobbying work, his finances, and (now) attempted witness tampering.

The most common theory for what’s really going on — floated by, among others, a federal judge — is that Mueller’s primary aim is to put pressure on Manafort with these other charges, so he’ll “flip” against President Trump on collusion or other topics.

Yet that theory raises other questions too. What could Manafort provide if he flips? Does Mueller need Manafort to flip, or can he make his case some other way?

Finally, why has Manafort been so hesitant to flip so far — and what might change his mind?

1) What is Mueller trying to achieve?

Mueller and his team have been remarkably close-lipped throughout their investigation — keeping President Trump’s team, the media, and Washington generally in the dark both about their overall strategy and about what their next moves will be.

Yet Mueller has made it clear in a court filing that last year, he was specifically authorized to investigate Manafort for potential criminal collusion with Russian government officials to affect the 2016 campaign.

Yet Mueller has not yet brought any charges from that part of the investigation, and has charged Manafort with other matters instead. Why?

There are a few possibilities. Perhaps Mueller feels he doesn’t yet have the necessary evidence for a collusion case. Perhaps Manafort wasn’t involved in collusion at all and there’s no case to be made. Or perhaps it’s as simple as Mueller’s trying to send Manafort to prison for repeated violations of the law and doesn’t care whether he flips or not.

The most widely believed explanation, though, is that Mueller is hoping to put pressure on Manafort to cut a plea deal so he’d provide information to them.

This is, after all, exactly what Mueller did with Manafort’s longtime business associate Rick Gates — charged him with a long list of crimes related to his Ukraine work and finances, but withdrew nearly all of those charges as soon as Gates agreed to flip.

But if we do take it for granted that Mueller is trying to flip Manafort, several other questions then arise about the special counsel’s strategy.

What, exactly, is Mueller hoping to get from Manafort, and why is he so convinced Manafort has that information? Does he think Manafort has a collusion “smoking gun” of some kind?

Manafort was at the top of the campaign, so who is Mueller trying to get him to flip on — President Trump himself? Other people close to Trump? Or maybe even certain Russians?

If Manafort does flip and provide incriminating testimony, why would anyone believe it, considering everything he’s been accused of already, and that he’d be blatantly trying to save his own skin? Is Mueller hoping to file charges against someone else in a court of law — or instead just to file a report on President Trump’s conduct to Congress, for potential impeachment?

How important, then, is flipping Manafort to Mueller’s case? Paul Rosenzweig recently speculated that the recent effort to send Manafort to jail before his trial could be happening because Mueller badly needs his cooperation, quickly. (Alternatively, perhaps Mueller has other ways to make his case, and he moved against Manafort because the new witness-tampering information just came to his team’s attention.)

2) What does Manafort know?

Related to all this is the question of what, if anything, Manafort actually knows regarding collusion between the Trump campaign and the Russian government during the 2016 campaign.

Manafort, of course, has said the answer is nothing. That no collusion happened, so he would naturally have no information on this to provide.

But though we can’t say for sure, it seems likely that Mueller believes otherwise. And there are two curious happenings during the campaign in particular that Manafort is involved in.

The Trump Tower meeting: For one, there’s that infamous meeting at Trump Tower that Donald Trump Jr. set up in June 2016, with a Russian lawyer and other Russia-tied figures. The three attendees of that meeting on the Trump side were Don Jr., Jared Kushner, and Manafort. No attendee has become a cooperator for Mueller. Perhaps the special counsel does think more remains to be learned about this meeting and hopes Manafort will tell him about it.

Oleg Deripaska and Konstantin Kilimnik: Perhaps even more suspicious are Manafort’s surreptitious contacts with two Russian nationals during the campaign. There’s his former client, the oligarch Oleg Deripaska, to whom Manafort was heavily indebted. And there’s Manafort’s longtime business associate Konstantin Kilimnik, who Mueller’s team has said is tied to Russian intelligence.

Just weeks after joining the Trump campaign, Manafort seemed to see an opportunity. He emailed Kilimnik in early April about his newly high media profile, writing, “How do we use to get whole,” and “Has OVD operation seen?” (Those are Deripaska’s initials.)

Then in July, Manafort and Kilimnik exchanged emails about Deripaska again, as the Washington Post and the Atlantic reported last year. “I am carefully optimistic on the issue of our biggest interest,” Kilimnik said. “He will be most likely looking for ways to reach out to you pretty soon.” Manafort wrote that if Deripaska “needs private briefings we can accommodate.”

The pair’s emails on the topic grew vaguer and more cryptic as the summer continued. In late July, Kilimnik wrote to Manafort, “I met today with the guy who gave you your biggest black caviar jar several years ago. We spent about 5 hours talking about his story, and I have several important messages from him to you.” This, again, is believed to be about Deripaska, with “caviar” thought to be code for money.

Kilimnik and Manafort arranged a meeting in New York City to discuss the matter on August 2 — Kilimnik wrote that he had a “long caviar story” to tell and “several important messages.”

Days after the meeting, Deripaska took a yacht trip with Sergei Prikhodko, Russia’s deputy prime minister, who is focused on foreign policy. (Oddly enough, the yacht trip was videotaped and posted to Instagram by an escort they’d invited along.) Again, all of this occurred while Manafort was chairing the Trump campaign, before his mid-August 2016 firing.

Now, this year, Kilimnik seems keenly interested in keeping Manafort out of jail — he was indicted alongside Manafort for obstruction of justice last week, for allegedly trying to get witnesses to give a false story. Yet Kilimnik is unlikely to ever face those charges, since he’s currently based in Moscow.

So what happened between Manafort, Kilimnik, and Deripaska during the campaign? We still don’t know. Maybe this where the action on Trump campaign/Russia collusion happened. Or maybe Manafort was just freelancing and trying to get himself paid. But it’s one of the biggest loose ends about what happened in 2016.

3) Why hasn’t Manafort flipped?

George Papadopoulos flipped. Michael Flynn flipped. Rick Gates flipped. Michael Cohen may be preparing to flip.

But Manafort has refused to do so. Even though he’s facing 25 charges that could easily put him in prison for the rest of his life — and even though the evidence for many if not most of these charges appears quite strong — he’s pleaded not guilty to everything, and has given no public indication he’s considering flipping.

Why? Again, one possible explanation is he just has nothing to flip with. Either he wasn’t involved in collusion or he has nothing on Trump or anybody else Mueller cares about. Alternatively, maybe Manafort is just holding out hope that he can beat the charges.

A darker possibility is that, given an apparent series of Russia-linked assassinations in the West, Manafort fears violent reprisals against himself or his family should he give information implicating Russians.

Others believe Manafort is just holding out for Trump to pardon him. The New York Times has reported that Trump’s lawyer John Dowd discussed a possible presidential pardon with Manafort’s lawyer last year, and Rudy Giuliani floated the idea again this Friday.

The problem there is that it’s not clear when Trump will feel comfortable giving that pardon, considering there are a host of complications involved, from upcoming elections to obstruction of justice concerns to potential state charges to the possibility that Manafort would no longer be able to avoid testimony by pleading the Fifth on certain matters.

In any case, Manafort’s life is about to become far more unpleasant now that he’s been sent to jail to await trial, with little apparent prospect of being released for months. Perhaps the high-flying multimillionaire will tire of his new lifestyle and reassess the wisdom of flipping. But what that would mean for Mueller’s larger effort remains unclear.