The office of special counsel Robert Mueller has charged Rick Gates, the former deputy chairman of President Donald Trump's campaign, on two counts related to conspiracy against the US and making a false statement to the FBI.

Gates pleaded guilty to both charges on Friday, indicating that he is cooperating with the investigation, which is probing the extent of Russia's interference in the 2016 US presidential election.

Gates is the fourth person known to be cooperating with Mueller.

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Rick Gates, the former deputy chairman of the Trump campaign, pleaded guilty on Friday to charges brought on by special counsel Robert Mueller, who is investigating Russian interference in the 2016 US election.

Mueller's office charged Gates with two counts related to conspiracy against the US and making a false statement to federal investigators. On Thursday, Mueller filed 32 charges against Gates and Paul Manafort, the former chairman of the Trump campaign, in a superseding indictment, accusing them of financial crimes related to tax and bank fraud.

Both men were first indicted last October on 12 counts related to money laundering, conspiracy against the US, and failure to register as foreign agents.

Gates said in a letter to his loved ones earlier Friday that he decided to plead guilty for the sake of his family, according to a letter obtained by ABC News.

Gates reportedly wrote: "Despite my initial desire to vigorously defend myself, I have had a change of heart. The reality of how long this legal process will likely take, the cost, and the circus-like atmosphere of an anticipated trial are too much. I will better serve my family moving forward by exiting this process."

"The consequence is the public humiliation, which at this moment seems like a small price to pay for what our children would have to endure otherwise," the letter read.

Gates is a close associate of Manafort, who has been a subject of intense scrutiny in the Russia investigation.

Gates' guilty plea makes him the fourth person known to be cooperating with Mueller, in addition to early campaign foreign-policy adviser George Papadopoulos, former national-security adviser Michael Flynn, and California businessman Richard Pinedo.

Thursday's charges against Gates and Manafort alleged that more than $75 million flowed through the defendants' offshore accounts and that Manafort laundered over $30 million in undisclosed income with Gates' assistance. Gates is alleged to have laundered more than $3 million.

Friday's filing made similar allegations with one noticeable change: it accused Manafort of laundering $18 million in undisclosed income, not $30 million.

The new charges allege that:

From 2006 to 2017, Gates “conspired to defraud the United States by impeding, impairing, obstructing, and defeating” investigations by US officials

When asked about a 2013 meeting between Manafort, a lobbyist and an unidentified member of Congress, Gates said there were no discussions about Ukraine when in fact there were.

Gates has often been described as Manafort's "right-hand man" and an influential Trump booster with wide-ranging connections to powerful leaders and businessmen around the world. He has been associated with Manafort for nearly three decades, and his testimony against his former boss could throw a wrench into Manafort's defense strategy.

Gates joined the Trump campaign in early 2016, when Manafort became the campaign chairman, and worked under him as his deputy. Manafort was forced to step down as Trump's campaign chairman in August 2016, but Gates stayed and worked on Trump's transition team. He was ousted from a pro-Trump lobbying group in April 2017 amid questions about Russia's election interference, but he continued to visit the White House as late as June, according to The Daily Beast.

By cooperating with Mueller, Gates can expect "a substantial reduction in his sentence," the Los Angeles Times reported, adding that he could serve up to 18 months in prison.

Paul Manafort leaves Federal District Court in Washington, October 30, 2017. AP Photo/Alex Brandon

The charges against Gates and Manafort stem from their financial dealings and their work as political consultants for Ukraine's pro-Russia Party of Regions and former Ukrainian President Viktor Yanukovych.

When Trump hired Manafort in March 2016, Manafort was known for having worked as a top consultant to Yanukovych, and he is widely credited with helping Yanukovych win Ukraine's presidency in 2010.

Yanukovych was ousted from the presidency in 2014 amid widespread protests against his Russia-friendly positions and his decision to back out of a deal that would have promoted closer ties between Ukraine and the West and distanced Ukraine from Russia. Yanukovych fled to Russia as the demonstrations escalated and grew bloody, and he is now living under the protection of the Kremlin.

Since then, Ukrainian prosecutors investigating the protests have said Yanukovych ordered security forces to open fire on protesters, and some are looking into whether Manafort had a hand in encouraging Yanukovych.

Manafort and Gates were also implicated in Mueller's investigation of Alex van der Zwaan, a Russia-linked lawyer who pleaded guilty earlier this week to one count of making false statements to investigators about his work for the law firm Skadden, Arps, Meagher, & Flom LLP and Affiliates in 2012.

At Yanukovych's request, Manafort asked Skadden to compile a report about the 2011 trial of the former Ukrainian Prime Minister Yulia Tymoshenko, an effort Van der Zwaan was involved in..

Tymoshenko, who was Yanukovych's rival in the country's 2010 election, was jailed in 2011 after the Ukrainian government convicted her of abuse of power. Her trial and subsequent conviction were criticized by human-rights groups and deemed politically motivated by the US, the UK, German, Italy, Spain, and other European countries.

Skadden was roped into the controversy over the Tymoshenko report when it emerged that the former Ukrainian Justice Minister Oleksandr Lavrynovych embezzled over $1 million to pay the law firm for its work.

Van der Zwaan was also accused of lying to investigators about why he did not provide Mueller's office with a September 2016 email between him and another person referred to in the charge as "Person A."

The court filing says Van der Zwaan not only spoke with both Gates and the unnamed person about Skadden's report on Tymoshenko's trial but also destroyed evidence Mueller's office was seeking, including the September 2016 email.

Read Friday's full court filing here: