Paul Manafort's deputy Rick Gates told a federal court Monday that he has revealed several other crimes to Robert Mueller's special counsel investigation beyond those his old boss is charged with.

The bombshell evidence will set off a new guessing game in Washington D.C. and beyond.

Gates was deputy to Manafort during his long-time boss's four months as Donald Trump's campaign chairman and was involved in all of his business dealings for years.

He was at Manafort's side at the 2016 Republican National Convention in Cleveland where Trump accepted the nomination, and was seen on stage as Ivanka Trump prepared for the speech she delivered before her father's acceptance speech.

Gates also revealed that the Mueller probe had said it would not oppose him if he applied for probation for the two crimes he pleaded guilty to in February - conspiracy against the United States and obstructing justice by lying to FBI agents.

The two could have seen him jailed for up to ten years.

After giving evidence on Monday he left the court room carrying a Dan Brown novel, His evidence was expected to resume on Tuesday and last up to three hours.

Stare: Paul Manafort (second from left) stared directly at his former deputy Rick Gates (right) as he gave evidence to the jury. Judge T. S. Ellis III, who had been a vocal critic of the prosecution, said little as prosecutors including Greg Andres questioned Gates

End of the day: Kathleen Manafort, Paul Manafort's wife (right) was accompanied by friends as she left the federal courthouse in Alexandria, VA, where Rick Gates will continue his evidence on Tuesday

Key place: Rick Gates (left) was there as Ivanka Trump practiced for her speech ahead of her father's acceptance speech in Cleveland, at the Republican National Convention

Turncoat: Rick Gates was Paul Manafort's long-term deputy, including during Manafort's four-month stint as Trump campaign chair, but is now to give evidence for Robert Mueller's prosecution of his old boss

Much of Manafort's business when Gates worked with him as his deputy was in Eastern Europe, where he became an adviser to Viktor Yanukovych, a powerful pro-Putin Ukrainian politician who became president of the country but was toppled after widespread protests.

They also worked for Oleg Deripaska, a Russian oligarch close to Putin who has not been mentioned at the trial, and who is subject to sanctions for his links to the Kremlin.

But the full extent of who Manafort dealt with and how remains unclear.

His rise to prominence goes back to the 1970s and Gates testified that he had known Manafort since 1995, when he began as an intern at his lobbying company, then called Black, Manafort, Stone and Kelly.

Manafort left that company around that time, but Gates was reunited with him around ten years later when he went to work at Manafort's new firm, DMP International.

Gates's political work with Trump was not covered in court at all, where the judge has agreed to a ban on mention of the president's name in case it prejudices the jury in a heavily anti-Trump area against Manafort.

But after Manafort was removed from his role as Trump campaign chair in August 2016, Gates remained involved.

He is known to have been a fundraiser for America First Policies, a pro-Trump super PAC, and then was involved in the inauguration committee.

The Daily Beast reported that he then worked for Tom Barrack, a Trump friend who chaired the inauguration committee, and was only terminated in October 2017.

Gates, 46, who has already pleaded guilty to obstruction of justice and faces up to 10 years in prison, appeared in a plain black suit in federal court to testify at Manafort's tax fraud trial as the prosecution's star witness on Monday.

Prosecutor: Assistant U.S. Attorney Uzo Asonye (standing) - who is seconded to the Robert Mueller special counsel investigation - questioned Gates. He was seen at the trial last week addressing jurors

He was asked by Uzo Asonye, the prosecutor: 'Were you involved in any criminal activity with Mr Manafort?'

He replied: 'Yes.' Asonye then asked: 'Did you commit any crimes with Mr Manafort?' Gates again replied: 'Yes.'

As Manafort stared directly at him, Gates spoke bluntly about the criminal activity he claimed he carried out under orders from Manafort, who he first met at a Christmas party in 1995 while interning for the former Trump campaign chairman's lobbying company.

Gates also admitted that he personally embezzled hundreds of thousands of dollars from Manafort in recent years – without Manafort's knowledge – by filing phony expense reports to their company DMP International.

'At Mr. Manafort's request at different points in the years we didn't disclose the foreign bank accounts [to accountants],' said Gates.

'That was in order to reduce the taxable income on [Paul Manafort's] tax returns,' he added.

Evidence: Cynthia Laporta's testimony raised the stakes for Manafort, legal experts said. Testifying under immunity, she was the first witness to admit she knew accounting maneuvers Manafort and Gates requested of her were wrong and could be crimes

Gates said Manafort ordered him to lie to accountants, and directed him to hide the company's foreign bank accounts, which housed millions of dollars the firm had earned from Manafort's Ukrainian political consulting clients.

Gates also told prosecutors that he was aware his actions were illegal.

He said Manafort asked him to make wire transfers from their foreign bank accounts, which were primarily based in Cyprus. According to Gates, he and Manafort contolled 12 undisclosed accounts in Cyprus, two in the Grenadines, and one in the U.K.

The accounts held Manafort's income from political consulting clients in Ukraine, said Gates.

He also said he did not file Foreign Bank Account Reports for DMP International, even though he knew this was illegal.

'We did not submit the proper form,' said Gates.

Gates admitted to submitting false information that inflated DMP International's income in order to fraudulently obtain a loan from Citizens Bank.

Gates said these activities were all done with Manafort's approval – but he also admitted to keeping his Manafort in the dark while personally embezzling from the company.

Gates said he submitted false expense reports to DMP International on multiple occasions. He said he did not recall how much money he stole, but estimated that it was approximately 'several hundred thousand dollars.'

He also claimed he carried out financial fraud while working on an unrelated side business with a movie producer named Steve Brown, and said he inflated his personal income on his own credit card and mortgage applications.

Prosecutors said Gates disclosed these crimes to them during their approximately 20 meetings with the former political consultant over the past year. Gates is not being charged for these activities due to a plea agreement he reached with the government.

Gates told the court he also lied to the government about a meeting Manafort had with an unnamed U.S. member of congress. Because of this, Gates pleaded guilty to making false statements to the government. This increased his potential jail time from five years to 10 years.

According to Gates's plea agreement, the government will recommend that he face between 57 and 71 months in jail. If Gates keeps his side of the agreement, the government also agreed not to object to Gates's attorney's request for probation. Prosecutors also said they would not bring addition charges

In exchange, Gates promised prosecutors in the plea deal that he would turn over evidence, cooperate as a witness, and testify in court. He said he has turned over documents, computers and phones related to the case to the government.

Legal team: Thomas Zehnle (center), Manafort's attorney is trying to convince the jury that Gates was actually to blame

Arriving: Paul Manafort's wife Kathleen (right) arrived

As part of the plea agreement, the government has also dismissed a second indictment against Gates. This indictment was related to tax fraud and bank fraud he said he committed while working for Manafort.

Manafort's attorneys have argued Gates committed tax fraud without Manafort's knowledge.

'We're primarily here because of one man, and that man is Rick Gates,' said Manafort's attorney Thomas Zehnle.

Zehnle claimed Gates 'embezzled millions' from Manafort by paying himself fake bonuses and expenses out of their company's bank accounts, and ultimately falsified company financial documents to avoid discovery.

'Rick Gates had his hand in the cookie jar, and he couldn't let his boss find out,' claimed Zehnle.

Zehnle argued Gates was trying to pin blame on Manafort because he wanted to avoid jail time and paying back taxes.

Prosecutors, and their witnesses over the past week, have countered that Manafort was fully aware he was defrauding the federal government, and intentionally lied and omitted information from his tax and loan documents.

Manafort's former accountant Philip Ayliff and his book keeper Heather Washkuhn told the court that Manafort was definitely in charge in his business relationship with Gates.

The federal trial jury in Alexandria, Virginia, had heard testimony on Friday from Cynthia Laporta, who described how Manafort and his associate Gates doctored financial statements and backdated loans.

Manafort has pleaded not guilty to 18 counts of bank and tax fraud and failing to disclose foreign bank accounts.

The charges largely predate his five months on the Trump campaign but arose from U.S. Special Counsel Robert Mueller's investigation into Russian interference in the 2016 U.S. election.

Manafort lawyer Kevin Downing said Gates would be the next witness called. Prosecutor Greg Andres walked into the courtroom with a binder labeled 'Richard Gates' earlier on Monday, the fifth day of the trial.

Since the trial started last Tuesday, Manafort's lawyers have kept their cross-examinations brief and at times refrained from attempting to rebut damaging testimony in detail.

But Laporta's testimony raised the stakes for Manafort, legal experts said. Testifying under immunity, she was the first witness to admit she knew accounting maneuvers Manafort and Gates requested of her were wrong and could be crimes. One accounting trick saved Manafort $500,000 in taxes, she said.

Laporta was the 14th witness called by prosecutors before U.S. District Judge T.S. Ellis.

Mueller is also investigating possible coordination between Trump campaign members and Russian officials, but the charges against Manafort do not address that.

Laporta detailed multiple examples in which Manafort and Gates sought to doctor financial records, first in order to lower Manafort's taxable income and then later to inflate his income so that he could get bank loans.

Some of the maneuvers were at the request of Gates, while others implicated Manafort, Laporta testified. In one instance Manafort's signature was on a loan agreement created in 2015 and inappropriately backdated for the 2014 tax year, she said.

Similar to prior witnesses, Laporta testified that Gates and Manafort were in lockstep but that Manafort was in charge.

The jury has heard how Manafort made tens of millions of dollars for political work with pro-Russian politicians in Ukraine.

Shanlon Wu, a former attorney for Gates before he pleaded guilty in February and starting cooperating with Mueller, said Monday would be a critical test of the defense's strategy of pinning the wrongdoing on Gates.

'They must make headway in separating Gates and Manafort,' Wu said. Wu said he was speaking as an observer now that he is no longer involved in the case.