Former Trump campaign chair Paul Manafort has agreed to cooperate with special counsel Robert Mueller’s prosecutors, after pleading guilty to crimes including money-laundering and conspiracy against the United States.

The plea deal, which includes a 17-page cooperation agreement, marks another victory for Mueller’s probe into Russia’s interference in the 2016 election and contacts with the Trump campaign. Manafort was previously convicted on tax and bank fraud charges in a separate trial last month. Given Manafort’s long-held ties to pro-Russian oligarchs and their associates in Ukraine, he’s likely to have considerable insight into any ties between the Trump campaign and the Kremlin.

The fact that Mueller’s team was willing to make a deal suggests Manafort had information that was valuable to the probe. Here are four topics they will doubtless want to explore.

The Trump Tower meeting

Manafort was one of three American attendees at a notorious meeting with a lawyer linked to the Russian government in Trump Tower. In the run-up to the meeting, Donald Trump Jr. was promised (paywall) he would be given documents that would “incriminate” Hillary Clinton, as “part of Russia and its government’s support for Mr Trump.”

The Trump team has insisted that the meeting was a nothing but waste of time, with Russian lawyer Natalia Veselnitskaya spending most of the time talking about Russian adoption by American families. In congressional testimony about the meeting, Don Jr. said he “didn’t know” or “couldn’t remember” 186 times. Testimony from Manafort could clear up several things that slipped the president’s son’s mind:

What did the meeting participants actually discuss?

Could Manafort explain his cyrptic notes from the meeting?

Did the Russians meet with Donald Trump, as Steve Bannon has suggested?

Why did Don Jr. speak to Emin Agalarov, the son of the Russian oligarch who helped arrange the chat, two times on the phone after the meeting?

Did Don. Jr speak to his father after the meeting? He received a call from a blocked number in between the two calls— and Trump is known to use a blocked number.

The RNC convention’s Ukraine platform changes

Many eyebrows were raised when the 2016 Republican National Convention softened an amendment supporting Ukraine, as the party prepared to make Trump its presidential nominee. A delegate from Texas wanted to add language that would support arming Ukrainians who were fighting Russia-backed forces in the East of the country, and also pushed for better coordination with NATO to stem Russia’s assaults. The ultimately deleted a reference to providing “lethal defensive weapons,” instead calling only for “appropriate assistance.”

Trump has insisted he had nothing to do with the language change, and other Republicans have said the move followed the proper procedure. But many are suspicious about Manafort’s role running the convention and his long history of working for pro-Russia forces in Ukraine—including his former patron, ex-Ukrainian president Viktor Yanukovych.

Robert Mueller included the event in his questions for Trump obtained by the New York Times, and will no doubt want to hear Manafort’s side of the story.

What exactly happened with Oleg Deripaska?

Paul Manafort has a long and uncomfortable history with Oleg Deripaska, a powerful Russian oligarch essentially seen by the US government as an arm of the Kremlin.

When Manafort joined the Trump campaign, Deripaska had long been chasing him over an $18.9 million debt. One of Manafort’s aides soon sent Deripaska’s team an email, asking how they can use his new-found power to “get whole” with the oligarch, and he reportedly offered the billionaire “private briefings” (paywall) about the Trump campaign. There’s no evidence to show that Deripaska accepted the offer, but briefing Deripaska would essentially mean briefing the Kremlin. Mueller will doubtless want to know if Deripaska was used as a backchannel to Moscow.

Did Trump offer him a pardon?

Many have suspected that Manafort held out against the temptation to take a plea deal for so long in the hope that Trump could pardon him. Harvard legal scholar Alex Whiting has argued that if Trump dangled such an offer in front of him, that could constitute obstruction of justice—one of the lines of enquiry Mueller has been exploring. Fellow law professor Ryan Goodman suggests the following question: