WASHINGTON (AP) — Alex van der Zwaan, an attorney, pleaded guilty on Tuesday to lying to federal investigators about his contacts with a Trump campaign official. Van der Zwaan worked for the international law firm Skadden, Arps, Slate, Meagher & Flom, which was hired to write a 2012 report on the Ukrainian government's prosecution of an opposition leader on corruption charges.

Former Trump campaign officials Paul Manafort and Rick Gates, who also have been charged as part of special counsel Robert Mueller's investigation, are accused of wrongdoing in connection with that report.

Here's an explanation of the work Skadden did — and how it ties into the Mueller probe.

Q: Why was Skadden hired to write a report on Ukrainian opposition leader Yulia Tymoshenko?

A: After winning the presidency in 2010 with Manafort's help, Viktor Yanukovych presided over the prosecution of some of his leading political opponents, including Tymoshenko, a former prime minister. The prosecution undercut Yanukovych's international standing because Tymoshenko was popular in the West and because her case had clear political undertones. So Yanukovych's Party of Regions began a charm offensive with Manafort's covert assistance.

Manafort and his assistant Rick Gates hired law and lobby firms to burnish the Yanukovych government's image — with a special focus on undercutting support for Tymoshenko. To help make the case that her prosecution was legitimate, Manafort and Ukraine's government hired a team from Skadden Arps to evaluate her prosecution.

Q: What did the report conclude?

A: Led by former Obama White House general counsel Greg Craig, the 303-page report concluded that some aspects of Tymoshenko's prosecution were flawed, but that it was not a travesty of justice. Although the report was at times critical of Ukraine's government, it concluded there was a plausible case against her.

Q: Who paid for the Skadden report and how much did it cost?

A: The answer wasn't — and still isn't — clear. Originally, Ukraine's Justice Ministry said it had paid just $13,000 for the report. According to Mueller's indictments last year of Manafort and Gates, the men used offshore bank accounts "to funnel $4 million to pay secretly for the report."

Separately, Skadden refunded $567,000 to the Ukrainian government last summer, saying the money had been prepaid by Ukraine's former government for work that was never performed. Why that refund was made more than three years after Yanukovych was ousted — and at a time when Mueller's team was scrutinizing Manafort's and Gates's work —remains murky.

Skadden has said it is cooperating with the investigation.

Q: Did Skadden ever register as a foreign agent in relation to its work and the promotion of the report?

A: No.

Q: Why would Mueller's team care about Skadden's work?

A: Part of Mueller's case against Manafort and Gates involves alleged money laundering and attempts to influence Washington on behalf of foreign clients. Van der Zwaan pleaded guilty to making false statements about his interactions with Gates.

Q: How else did Skadden get involved with Manafort and Gates?

A: To hide their Washington lobbying work on behalf of Ukraine's government, Manafort and Gates passed money through a group in Brussels to pay two lobbying firms, Mercury Public Affairs and the Podesta Group, according to Mueller's indictments.

The organization, called the European Centre for a Modern Ukraine, had ties to Ukraine's government, and Manafort and Gates were directly involved in supervising its work. But Skadden partner Ken Gross signed off on Mercury Public Affairs' decision not to register itself as a foreign agent when doing the lobbying work — a move that would have exposed the firm's activities to more public scrutiny. Among the lobbying firms' tasks was to undercut public support for Tymoshenko, according to emails obtained by the AP.