Paul Manafort’s former business partner Rick Gates testified on Monday as a witness against Manafort, as part of Special Counsel Robert Mueller’s first case to go to trial, largely involving alleged financial crimes before either joined the Trump campaign.

Gates, who testified against his former mentor in exchange for a plea bargain, testified that he committed crimes with Manafort and admitted he stole money from Manafort, as well as from others. He testified that at Manafort’s instruction, he lied to Manafort’s accountants about foreign bank accounts in order to avoid paying taxes on that money.

He also said Manafort helped four Ukrainian businessman elect a pro-Russia president in Ukraine in 2010 in exchange for millions that were paid to Manafort through shell companies. Gates also testified that a Russian citizen, who is allegedly tied to Russian intelligence, had signatory authority over some of Manafort’s hidden accounts in Cyprus, according to reports.

Mueller’s team hopes Gates’ testimony will lead to the conviction of Manafort for a list of financial-related charges that include: conspiracy against the United States, conspiracy to launder money, failure to file reports of foreign bank and financial accounts, being an unregistered agent of a foreign principal, false and misleading FARA statements, and false statements. Manafort has pleaded not guilty.

Manafort’s defense team aimed to discredit Gates as a liar and an embezzler who falsified Manafort’s finances while trying to cover up his own crimes. Gates admitted to submitting inflated expense reports.

Gates, 46, as part of his plea deal, pleaded guilty to lying to federal authorities and conspiracy to commit fraud, which could carry up to six years in prison. Manafort, 69, meanwhile, is facing 18 felony charges that could carry a maximum of 30 years in prison.

Gates is considered the “star witness” of the trial, which is in its second week. During the first week, the Special Counsel team portrayed Manafort as a flashy profligate who lived an excessively lavish lifestyle. Prosecutors displayed picture after picture of Manafort’s extensive wardrobe, including a gaudy ostrich-skin jacket.

The trial comes 14 months after Mueller was appointed, and after it has spent at least $17 million.

Gates began working for Manafort in 2006 on his business ventures in Ukraine. According to the New York Times:

Mr. Manafort provided the connections and signed off on all the big decisions, but it was Mr. Gates who devised much of the strategy and administered the business, according to people who worked with the pair there. They said Mr. Gates did everything from approve expense reimbursements to draft business plans.

Manafort joined the Trump campaign on March 26, 2016, as the campaign’s GOP convention manager. By that time, he had worked on several Republican presidential campaigns, including those of Gerald Ford, Ronald Reagan, George H.W. Bush, and Bob Dole.

Manafort served as candidate Donald Trump’s campaign chairman from May 2016 through August 2016. Gates came along with Manafort and managed to snag a different campaign job after Manafort left, as the campaign’s liaison to the RNC. Trump reportedly disliked Gates.