Rick Gates is considered Robert Mueller’s star witness in the federal government’s case against former Trump campaign chairman Paul Manafort. And on Monday he showed why: implicating his longtime partner in a series of crimes that could put Manafort behind bars for the rest of his life.

How’d he do it? By testifying under oath that he was right there committing the crimes with him.

Taking the witness stand before a packed courtroom, Gates said he helped Manafort conceal millions the pair earned together while working as political consultants in Ukraine during the years before they both signed up to work on President Trump’s 2016 presidential campaign.

Moments after taking the stand, Gates admitted that he:

Assisted Manafort with filing false tax returns

Assisted Manafort in hiding 15 foreign accounts from the federal government

Provided fraudulent and altered documents to financial professionals

Lied to the accountants working for Manafort personally and their consultancy

Arbitrarily identified incoming financial flows as loans instead of income to reduce Manafort’s tax bill

With his arms crossed, Manafort stared directly at Gates with a look of smouldering intensity. During breaks in the testimony, Gates looked straight ahead or down at his hands, and didn’t meet Manafort’s gaze. Gates spoke in calm, quick, measured sentences — appearing deeply familiar with the facts laid out in the case.

From day one, Manafort’s team of lawyers have sought to pin all the blame for any financial impropriety on Gates, presenting him as a serial liar who betrayed his friend and partner in the process. Central to that character outline was the allegation that Gates had gone so far as to embezzle money from Manafort.

On Monday, Gates put that question to rest by admitting to stealing hundreds of thousands of dollars from his former boss by filing false expense reports.

“Did he know they were fake?,” asked prosecutor Greg Andres.

“No,” Gates replied.

Rick Gates, former campaign aide to U.S. President Donald Trump, departs after a bond hearing at U.S. District Court in Washington, U.S., December 11, 2017. REUTERS/Joshua Roberts

Gates, who has already pled guilty to counts of conspiracy and lying to investigators, seemed to leave no stone unturned in his sweeping testimony, even admitting to once returning 15 minutes late to 11:00 pm house arrest curfew after reaching a plea-deal with the government.

That deal includes a promise to testify, if needed, in trials like the one against Manafort. And in front of the jury on Monday, prosecutors sought to demonstrate that Gates has a powerful incentive to tell the truth now, by pointing out he faces grave legal jeopardy if he’s found to be lying after reaching an agreement not to do so.

“If you violate the agreement, what happens?,” Andres asked.

“I lose all the benefits associated with the guilty plea,” Gates replied.

But in deciding to put Gates on the stand Monday — a move legal observers have said was far from certain — the team led by special counsel Robert Mueller may be taking a risk. Manafort’s defense attorneys have signaled they plan to take a blowtorch to Gates’ credibility. If they are successful, that could impact Gates’ ability to testify for Mueller’s team in other cases going forward about other aspects of their investigation he may know about.

As the trial closed for the day, prosecutors indicated that despite the laundry list of wrongdoing Gates had just admitted to, he was only getting started. Andres, the prosecutor, said Gates would take the stand again first thing Tuesday morning — with further testimony expected to last for three more hours.