The near-daily barrage of news and revelations, big and small, about the Trump campaign and its metastasizing ties to Russia can be hard to keep track of, even for people following the scandal closely. Story lines and players appear and disappear, sometimes for weeks or even months at a time.

While there remain big, overarching questions about whether there was active conspiracy between Trump, his associates, and Russia—or merely opportunistic collusion—the answers to those questions could be amorphous and long in coming.

More simply and immediately, there’s plenty of information that we know we don’t yet know about what went on in the campaign, from cyber meddling to clandestine meetings surveilled by US and other intelligence agencies—missing puzzle pieces that we can discern from the revelations that have come out so far. As Donald Rumsfeld famously said in the early days of the Iraq War, “As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say, we know there are some things we do not know.”

As the Trump campaign’s onetime chairman Paul Manafort—a key figure in the scandal—makes his way toward the Senate Judiciary Committee, we thought the time was right to, in Rumsfeldian terms, present a nonexhaustive list of 15 of the most pressing known unknowns in the Trump/Russia investigation: holes and unanswered questions that you can bet Special Counsel Robert Mueller is digging into.

1. What was said on the Kislyak intercepts?

We know that something in last December’s telephone calls between Russian Ambassador Sergey Kislyak and soon-to-be Trump national security adviser Michael Flynn triggered concern among US intelligence. Presumably the calls focused on the sanctions against Russia that the outgoing Obama administration had imposed at that time. What was said, and by whom, that led US intelligence to begin the lengthy and complicated process of flagging the conversations as concerning?

2. Are there intercepts discussing the Kislyak “backchannel” discussion?

We know that there are US intelligence intercepts—either of telephone calls or electronic communication—that contradict Jeff Sessions’ recounting of his interactions with Sergey Kislyak. This week, presidential adviser and Trump son-in-law Jared Kushner said that the so-called secret backchannel offer was simply a means of attempting to gain reliable information on the Russian military’s view of the Syrian conflict. Is Jared Kushner’s reporting of that meeting consistent with any Russian reports of the meeting—and did the US or its allies intercept those reports as they were dispatched to Moscow?

3. Intelligence agencies in Europe and among our so-called Five Eyes partners— Australia, Canada, New Zealand, and the United Kingdom—had evidence of suspicious Trump interactions in 2015 and 2016. What triggered them?

News reports have indicated that the first alerts to the US government of suspicious “interactions” between Trump associates and people associated with Russian intelligence came from European intelligence partners in late 2015. According to the Guardian, intelligence and tips came from a wide variety of European and Five Eyes partners, which form the world’s closest and deepest intelligence alliance. Tips gathered during routine surveillance of Russian intelligence assets evidently filtered into the US from countries as widely sourced as Germany, Estonia, Poland, and the Netherlands, and from the French foreign intelligence service, DGSE. What were they seeing and what were they warning about?

4. Relatedly, where was the Russian money going?

According to news reports, last April CIA director John Brennan was given a tape recording, allegedly from a friendly Baltic intelligence service, which purported to indicate that Russian—perhaps even Kremlin—money was being funneled to the Trump campaign. Where was that money coming from and where was it purportedly going?