Trump-Russia special counsel Robert Mueller is authorized to investigate "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump." The popular word for that is collusion, and it remains at the heart of both the Mueller and the Senate Intelligence Committee investigations. (Majority Republicans on the House Intelligence Committee recently announced they were unable to find evidence proving collusion.)

While much about the Mueller investigation remains unknown, we do know the indictments he has filed and the pleas he has reached with various figures in the case. There are some, including charges against Russia's Internet Research Agency, several individual Russians, and two minor figures, that target either people on the Russian side of the equation or those for whom there are no suspicions of collusion.

But Mueller has charged three people who were in the Trump campaign inner circle -- former national security adviser Michael Flynn, former campaign chairman Paul Manafort, and former deputy campaign chairman Richard Gates, all with ties to Russia and all of whom might be expected to be part of a collusion scheme, had one existed. Mueller has also charged one peripheral hanger-on, George Papadopoulos, who might conceivably have been part of a collusion scenario.

But all have faced charges and none of those charges, at least so far, has involved allegations that Flynn, Manafort, Gates, or Papadapoulos played a role in a scheme of collusion, or coordination, or conspiracy, or whatever it is called. And that could tell the public something about the state of the collusion allegation inside the Mueller investigation.

To put it briefly: What kind of collusion scheme between Russia and the Trump campaign could have existed without Michael Flynn being part of it? What kind of collusion scheme could have existed without Manafort? And Gates? And yet none of them -- nor Papadapoulos, either -- has been charged with taking part in a collusion scheme.

Flynn pleaded guilty to one count of lying to investigators about what he discussed in a phone conversation with Russian ambassador Sergey Kislyak. (Investigators have always known what the two men talked about, since the call was wiretapped, recorded, and transcribed.)

Manafort has been charged with a daunting number of financial crimes, including fraud and tax evasion, relating to his work for a pro-Russian political party in Ukraine.

Gates was charged with most of the crimes Manafort faced, but received a deal from Mueller, allowing him to plead guilty to one count of lying to investigators and one count of hiding profits from his work with Manafort in Ukraine.

Both Gates and Manafort were charged with something called "conspiracy against the United States." As the former prosecutor Andrew McCarthy points out, "there is no such offense in federal law as 'conspiracy against the United States.'" But Mueller's office described their alleged crimes that way, and some media commentators liked the treason-ish sound of the phrase.

Papadopoulos pleaded guilty to one count of lying to investigators about a conversation he had with a Russian who wanted to create ties with the Trump campaign.

All those charges, and none alleging that any of these men, some of them at the highest levels of the Trump campaign, took part in any collusion, or coordination, or conspiracy between the campaign and Russia.

"I can't imagine that there would have been collusion or conspiracy with the Russians that Michael Flynn didn't know about," said Sol Wisenberg, a former prosecutor with the office of independent counsel Kenneth Starr. "If you're trying to make a collusion case and you are Mueller, you're trying to get someone to plead to the crime you're trying to prove."

McCarthy has written much the same. "When a prosecutor has a cooperator who was an accomplice in a major criminal scheme, the cooperator is made to plead guilty to the scheme," he wrote last December. "This is critical because it proves the existence of the scheme."

Even Preet Bharara, the New York U.S. attorney fired by Trump and no fan of the president's, said last year, "When we had evidence against somebody and wanted them to flip, we made them plead guilty to every bad act that they had ever done, especially if we were later going to be alleging other people had engaged in that activity as well."

But in the Mueller investigation, it is precisely the people who would most be expected to be part of a collusion scheme who have not been charged with taking part in any such activity.

Nevertheless, for those hoping for collusion, there are still some possibilities. Mueller might lodge, or might have already lodged, additional charges against Flynn, Manafort, or Gates.

And there is still Carter Page, like Papadopoulos a sometime volunteer Trump adviser, who traveled to Russia in 2016. Page has been publicly vague about his dealings with Mueller but said recently that he has been interrogated for more than 30 hours in the last year by the executive and legislative branches of government. Perhaps Mueller is waiting to charge him with something. Or maybe someone else will be charged with taking part in the long-sought collusion.

Still, it's hard to imagine a collusion plot that never touched Flynn, Manafort, or Gates. Maybe it happened, but after more than 18 months of FBI, and now Mueller, investigations, it's becoming harder to see how.

