Open this photo in gallery Kevin Downing, left, and Thomas Zehnle, attorneys representing former Trump campaign chairman Paul Manafort, arrive for their client's trial as it continues in federal court in Alexandria, Va., on Tuesday, Aug. 7, 2018. Jacquelyn Martin/The Associated Press

President Donald Trump’s former campaign chairman Paul Manafort, who is on trial on tax and bank fraud charges, had US$16.5-million in unreported taxable business income between 2010 and 2014, a U.S. Internal Revenue Service agent testified on Wednesday.

IRS agent Michael Welch told a jury that Manafort’s unreported income includes foreign wire transfers to U.S. vendors like landscapers and clothiers, wire transfers to buy property, and income improperly reclassified as loans.

Welch’s testimony came as prosecutors sought to refocus the courtroom’s attention on Manafort’s alleged financial crimes after his defence attorneys spent hours trying to undermine the credibility of their star witness, former Manafort business partner Rick Gates.

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Welch said he arrived at the US$16.5-million figure based on an accounting method used by Manafort.

During his review, he said, he discovered that many of the foreign wire transfers did not appear on general ledgers for Manafort’s political consultancy and therefore, “I was not able to trace it into the tax return.”

Star government witness Rick Gates ended three days of testimony on Wednesday after admitting he lied, stole money and cheated as lawyers for U.S. President Donald Trump’s former campaign chairman Paul Manafort attacked his character.

As Mr. Gates took the witness stand for the final time at the tax and bank-fraud trial of his former boss, Mr. Manafort’s lawyer Kevin Downing asked if his “secret life” spanned the years 2010 to 2014.

“I have made many mistakes over many years,” Mr. Gates, 46, replied in federal court in Alexandria, Va.

Mr. Downing spent several hours on Tuesday firing questions at Mr. Gates to attack his credibility as a witness, bringing up lies, an extramarital affair in London he used the firm’s expenses to pay for and money he embezzled from Mr. Manafort. Mr. Downing also asked whether he submitted personal expenses when he worked for Mr. Trump’s inaugural committee.

Mr. Manafort is the first person to be tried on charges brought by special counsel Robert Mueller’s investigation into Russian interference in the 2016 U.S. presidential election. Mr. Manafort made millions of dollars working for pro-Russian Ukrainian politicians before he took an unpaid position with the Trump campaign that lasted five months.

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Mr. Gates, who pleaded guilty to charges in February and is co-operating for the possibility of a reduced sentence, testified that he helped Mr. Manafort falsify his tax returns, lie to banks to get loans and hide foreign bank accounts.

Mr. Gates, a married father of four, faced tough questioning under cross-examination on Tuesday about his leading a “secret life.”

“In essence, I was living beyond my means,” Mr. Gates said. “I’m taking responsibility for it. I made a mistake.”

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Prosecutors on Wednesday planned to call an FBI forensic expert who traced Mr. Manafort’s accounts.

Mr. Manafort, a long-time Republican political consultant, has pleaded not guilty to 18 counts of bank fraud, tax fraud and failing to disclose foreign bank accounts. According to testimony, he used the accounts to receive payments from Ukrainian oligarchs.

Mr. Manafort’s defense team’s has tried to pin much of the blame for financial crimes on Mr. Gates and raise doubts about his ethics and morals.

Prosecutor Greg Andres addressed the defence lawyer’s questions about whether the special counsel’s office had tried to coach Mr. Gates on how to testify.

“The only answer I was told was to tell the truth,” Mr. Gates replied.

Mr. Manafort’s conviction would undermine efforts by Mr. Trump and some Republican lawmakers to paint Mr. Mueller’s inquiry as a political witch hunt.

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Moreover, Mr. Manafort remains a central figure in the broader inquiry into the Trump campaign’s dealings with Russia, including a 2016 Trump Tower meeting at which Russians promised “dirt” on Democratic presidential candidate Hillary Clinton and his role in watering down the 2016 Republican Party platform position on Ukraine.

In detailed testimony, Mr. Gates walked prosecutors through the step-by-step process on how he and Mr. Manafort doctored and backdated documents.

In one example, Mr. Manafort and Mr. Gates e-mailed each other copies of a doctored profit and loss statement they later sent a bank to help Mr. Manafort obtain a loan.

Mr. Gates also admitted he covertly wired funds out of Mr. Manafort’s offshore accounts to line his own pockets, using the same tricks he deployed to help doctor and falsify records for Mr. Manafort.

Mr. Downing seized on those admissions to try and cast doubt on whether the jury can trust Mr. Gates’s claims that he only carried out the fraud alleged by prosecutors at Mr. Manafort’s behest.