There is so much about Special Counsel Robert Mueller’s investigation into Russian interference in the 2016 election that we just don’t know. Mueller and his team have access to intelligence intercepts, financial records, Trump campaign and transition emails, and hours upon hours of interviews with Trump campaign and White House officials. The special counsel’s office might not have a full picture yet of what happened during the 2016 election, but they certainly have far more of the puzzle pieces than the rest of us do.

Every time there is a major development in the investigation, we learn just how much we don’t know. For example, on the same day the charges against former Trump campaign chief Paul Manafort were revealed, the public also learned about George Papadopoulos, who up until that point was a little-known Trump campaign adviser who had multiple contacts with Russians linked to the Kremlin. Then came the plea agreement of retired Lt. Gen. Michael Flynn, President Donald Trump’s first national security adviser. It revealed that a handful of senior members of the transition team were not only aware of Flynn’s December 2016 calls with Russian Ambassador Sergey Kislyak, but had coordinated the talking points with him. This upended the White House’s narrative that Flynn had gone rogue and raised questions about the White House’s official story that Vice President Mike Pence had been kept in dark about the substance of Flynn’s calls. This bombshell in Flynn’s plea agreement has largely been forgotten in the flood of other news, but it recently led to KT McFarland withdrawing her nomination to be ambassador to Singapore, because now, thanks to Mueller, her lying has also been exposed.

So what are the really big things we still don’t know? In a series of posts, I’m going to examine the investigation’s central mysteries. To kick things off, I’ll start with the most important question:

Did the Trump campaign collude with Russia and did Trump himself participate or know about it?

Trump cannot say it enough: There was no collusion. In a December interview with the New York Times, he “insisted 16 times that there has been ‘no collusion’ discovered by the inquiry.”

The truth is, like the rest of us, the president has no idea what Mueller’s team has found over the course of its investigation. But while Mueller’s team toils away in secrecy, investigative reporters have also been digging over the past year and they repeatedly unearth new information that shows a clear pattern of Russian officials approaching various members of the Trump campaign and being met with open arms.

First, there’s Donald Trump Jr. After being offered dirt on Hillary Clinton from a Russian official in June 2016, Trump Jr. responded, “If it’s what you say I love it especially later in the summer.” Following that exchange, Trump Jr. set up a meeting in Trump Tower with a “Russian government attorney” and others with suspected ties to the Russian government. The meeting was also attended by Paul Manafort, a long-time associate of Trump and who was at the time the Trump campaign’s chairman, and Trump’s son-in-law Jared Kushner. When it became clear the New York Times was going to publicly disclose that the meeting took place, the president himself helped concoct a cover-up story, which was given to the Times.

Next, you’ve got George Papadopoulos, who served as a foreign policy adviser to the campaign. Despite being described as a “coffee boy,” by one former Trump campaign adviser, he remained influential throughout the race, helping to arrange a meeting between Trump and Egyptian President Abdel Fattah el-Sisi two months before the election. Papadopoulos was also photographed in a campaign meeting with Trump, during which the young adviser briefed the group that he had connections that could help arrange a meeting between Trump and Russian President Vladimir Putin, an idea that candidate Trump reportedly did not dismiss.

As Sen. Mark Warner, the Democratic ranking member on the Senate Intelligence Committee, put it, “I don’t know any coffee boy in any campaign that I’ve been involved with that had direct communications with the absolute senior leadership with the campaign.”

Whatever his position, Papadopoulos told an Australian diplomat in May 2016 that the Russians had political dirt on Clinton. When Australian officials passed this information on to the U.S. government, it triggered the FBI to open its investigation into Russian interference and whether the Trump campaign played any role in it. Papadopoulos’ plea agreement revealed how enthusiastically Papadopoulos was pursuing a relationship with the Kremlin on behalf of the Trump campaign; how a top Trump campaign official encouraged him in this endeavor, and how he later lied to FBI investigators about these interactions in an effort to cover them up.

Not to be overlooked is Manafort, whose true role in all of this is yet to be revealed. But we do know his ties to Kremlin-linked oligarchs stretched way back and was at the center of his years of work in Ukraine. While working on the Trump campaign, Manafort reportedly told an intermediary that “private briefings” could be arranged for his former client and Russian oligarch, Oleg Deripaska.

There’s also Kushner, who in December, discussed setting up “a secret and secure communications channel between Trump’s transition team and the Kremlin.” And multiple reports indicate that Flynn’s calls with Kislyak were at the direction of Kushner. And there is his national security questionnaire — known as an SF-86 — which Kushner has had to update multiple times because he failed to disclose multiple foreign contacts, including with Russian officials during the campaign. It wasn’t until Kushner filed a second addendum that he disclosed the June 9, 2016, Trump Tower meeting.

Finally, there’s Carter Page, who also served as a foreign policy adviser on the Trump campaign. In 2013, Page had been targeted by Russian spies working in New York City and was eventually confronted about his interactions with them by the FBI. In the spring of 2016, Trump named Page as one of his few foreign policy advisers in an interview with the Washington Post. While working for the campaign in July 2016, Page traveled to Moscow and met with Russian Deputy Prime Minister Arkady Dvorkovich and Andrey Baranov, Rosneft’s head of investor relations. And as the Nunes memo revealed, the FBI obtained permission from a Foreign Intelligence Surveillance Court to spy on Page not once, but four times, starting in October 2016. This means the Justice Department had to demonstrate probable cause to think that Page was “knowingly engaging in clandestine intelligence gathering activities for or on behalf of” Russia.

The pattern is quite clear: When Russian officials approached Trump campaign members, they were all too happy to oblige them. They took secret meetings with them; they offered information; they expressed enthusiasm at Russia’s offer to help them win the election. So, now we know Russian intelligence was taking every opportunity it could to infiltrate the campaign. And we also know that it met little resistance in its efforts to do so. But the question remains: Then what happened? Did members of the Trump campaign offer to help amplify the Russian government’s disinformation campaign? Did the Trump team promise pro-Russian policies (undoing sanctions, for example) should they win the election? Was their money or data exchanged? We just don’t know, but Mueller might.

We also don’t know what Trump himself knew and when he knew it. Was he aware of the Trump Tower meeting as it was happening? Did Papadopoulos’ knowledge in May 2016 that the Russians had dirt on Clinton make its way up the food chain to Trump? Did Trump, like the rest of his senior leadership team, know what Flynn was talking to Kislyak about?

With all of the this seemingly collusive behavior being exposed, the Trump White House has changed its messaging around the investigation. For most of 2016 and well into 2017, the message from the White House and other Trump officials was: There were no contacts with any Russian officials. In February 2017, Trump said, “I have nothing to do with Russia. To the best of my knowledge, no person that I deal with does.” As late as July 2017, Trump told the Wall Street Journal, “There’s nobody on the campaign that saw anybody from Russia. We had nothing to do with Russia.”

But of course, we now know this isn’t true. And once it became painfully clear how untrue this was, Trump’s defense switched to emphasizing: “There was no collusion.” This remains the go-to fallback line, but increasingly you hear from Trump and his team, “even if there was collusion, it’s not a crime.” Trump told the Times in December, “No. 1, there is no collusion, No. 2, collusion is not a crime, but even if it was a crime, there was no collusion.”

Trump’s continued insistence that there was no “collusion” at any level of his campaign, as evidence piles up to the contrary, is rightfully starting to raise some basic questions.

The Times’ Maggie Haberman asked White House Press Secretary Sarah Huckabee Sanders recently, “Can you define what he means when he says ‘collusion’? Is he talking about meetings between officials? Is he talking about information-exchanging hands? What does that mean?”

Sanders: I think the accusation against the President is that he had help winning the election, and that’s simply untrue. The President won because he was the better candidate, because he worked harder, because he had a message that America actually cared about and believed in, and came out in a historic fashion and supported and voted for him. That’s why he won. It wasn’t because of some made-up hoax that has been created to delegitimize this President. It’s because he was the best candidate, at the right time, that America wanted to see, and that’s why he’s in the Oval Office today. Haberman: Sarah, just to follow up. Does he think that the reporting from the intelligence community saying that there was hacking that went on, done by Russia — he rejects that or does he accept it? Sanders: No, he’s addressed that, but that doesn’t mean that he participated in it. I think those are very different things. Stating the existence of something happening is very different than having helped make it happen, and you can’t conflate the two. And I think oftentimes that’s what individuals are trying to do. Haberman: Right. Does he mean that about himself or about campaign officials? When he says collusion between the campaign, does he mean himself, or does he mean that no one on his campaign could have known anything? Sanders: Look, I think he’s stating for himself and to anything that he would be a part of, or know about, or have sanctioned. But that would be something that, again, I think he’s very clearly laid out he and his campaign had nothing to do with.

That members of the Trump campaign had nothing to do with Russia’s interference in the election is a statement standing on increasingly shaky ground. Whether Trump knew what his campaign advisers were up to is another question that hopefully time (and a little investigative reporting) will tell.

Image: Win McNamee/Getty