Special counsel Robert Mueller was appointed last May to oversee the probe into Russian interference in the 2016 presidential election. Since then, though, he’s charged former Trump campaign chair Paul Manafort with a series of crimes related to his financial activities and work in Ukrainian politics that largely happened before the election.

The reason for his focus on Manafort has long seemed obvious: Mueller was investigating potential collusion between the Trump team and Russia, and Manafort was a clear suspect there ( due to both his history of work for pro-Russian entities and the fact that he surreptitiously met and communicated with Russians during the campaign).

Still, Mueller has never outright said he was pursuing Manafort for potential collusion — until now.

A filing in Mueller’s case against Manafort in Washington, submitted late Monday night, provides some more specifics about just why the special counsel is so focused on Trump’s former campaign chair.

It reveals that back in August, Deputy Attorney General Rod Rosenstein specifically authorized Mueller to investigate whether Manafort committed crimes “by colluding with Russian government officials” to interfere with the 2016 election.

But Mueller gets more specific than that. He writes that in probing Manafort, he wanted:

To learn about his “ties” to and “interactions” with Russian and Russia-tied figures

To understand his “motives and opportunities to coordinate”

To “expose possible channels for surreptitious communications”

And, finally, to “follow the money trail”

Again, Manafort has not yet been charged with any crimes directly related to collusion with Russia. He has pleaded not guilty to the charges Mueller has filed against him so far.

Rosenstein authorized Mueller to investigate Manafort for potential Russia collusion crimes

The background here is that a while back, Manafort filed a motion to dismiss the charges against him. Among other arguments, he claimed that Mueller exceeded his authority as special counsel by investigating and charging Manafort with crimes unrelated to his central assignment: investigating Russian interference in the 2016 election. (Manafort was instead hit with a series of alleged tax, bank fraud, and false statement charges on matters that mostly predated the campaign, focusing on his Ukrainian work.)

Mueller had to respond and has done so in this new filing. Some of his arguments rely on Justice Department authorities and past precedents involving the special counsel regulation. And, importantly, when Rosenstein appointed Mueller, he also gave him the authority to pursue “any matters that arose or may arise directly” from the Russia investigation.

But the most interesting parts of Mueller’s new filing are when he makes the case that the Manafort prosecution is closely tied to his Russian interference mandate.

Mueller does this, first, by providing a document we’ve never seen before. Dated August 2, 2017, it is a letter from Rosenstein confirming that Mueller does have the authority to investigate allegations involving several specific individuals. (And, Rosenstein writes, that these investigations properly fell under Mueller’s mandate all along.)

You can read the letter at this link, but large blocks of it related to other people Mueller is investigating are redacted, with only a comparatively short section related to Manafort unredacted. Still, this is the confirmation that Mueller was investigating Manafort for collusion with “Russian government officials” about Russian “efforts to interfere” with the presidential election. (It also confirms that Rosenstein specifically approved Mueller looking into Manafort’s Ukraine money.)

So the letter confirms that Mueller was investigating Manafort for potentially colluding with Russia.

Seven months have passed since then, though, and no charges on that front have yet materialized. Did Mueller hit a wall? Did he conclude there was nothing there? Or is he still actively pursuing the matter? We don’t really know.

Mueller argues that Manafort’s Ukraine ties are naturally connected to the Russia probe

In his own newer filing, Mueller understandably avoids making any uncharged allegations about Manafort and collusion crimes. However, he does more specifically lay out his team’s thinking on why Manafort’s Ukrainian work is naturally connected to the question of Russian interference in the 2016 election.

Rolling his eyes a bit, Mueller writes that an investigation of whether the Trump team colluded with Russia “would naturally cover ties that a former Trump campaign manager had to Russian-associated political operatives, Russian-backed politicians, and Russian oligarchs.” (The “duh” there is implied.)

He goes on to provide some more specifics later in that same paragraph:

I count five specific reasons Mueller offers for his focus on Manafort. Here they are (in bold), with my own interpretations added afterward.

Investigating his ties to Russian and Russia-tied figures: Manafort’s Ukraine work was done for the pro-Russia political party and then-government of Viktor Yanukovych. He also, as Mueller mentions in this new filing, has worked for the Russian oligarch Oleg Deripaska. Mueller argues that these “ties” clearly merited investigation. Probing interactions with Russian and Russia-associated figures, both before and during the campaign: It’s been publicly reported that while working for Trump, Manafort tried to brief Deripaska about the campaign. Manafort also remained in contact with Konstantin Kilimnik, a longtime business associate who Mueller says had ties to a Russian intelligence service in 2016. Exploring “motives and opportunities” for Manafort to coordinate with Russia: Mueller is not specific here, but Manafort’s longtime business history with pro-Russian entities provides one obvious motive for coordination. Manafort’s finances provide another — he was heavily in debt, and to Deripaska specifically. In fact, shortly after Manafort joined Trump’s campaign, he emailed Kilimnik some positive articles about his new gig and asked, “How do we use to get whole?” Exposing “possible channels for surreptitious communication”: This one is particularly intriguing. We know that in summer 2016, Kilimnik emailed Manafort with what seems to be vague, coded language about Deripaska. Kilimnik said he had “several important messages” from Deripaska, and that he had a “long caviar story” to tell (“caviar” is thought to be code for money). On August 2, 2016, Kilimnik and Manafort met in New York City. A few days later, Deripaska took a yacht trip with Russia’s deputy prime minister. To “follow the money trail”: The classic Watergate line.

You can read the full new Mueller filing below, or at this link.