The ex-girlfriend and deputy of Sam Bankman-Fried, the founder of the crypto exchange FTX, has said he "directed" her to take customer's money to repay risky loans he had obtained.
Caroline Ellison was testifying in the trial of Mr Bankman-Fried, who is accused of fraud and money laundering.
Ms Ellison said her former boss directed a risky strategy that relied on the support of FTX deposits.
She said systems he set up led to billions being stolen from customers.
FTX was once one of the biggest platforms in the world where crypto investors could buy and trade digital currencies. It went bankrupt in November, with more than $8bn (£6.51bn) reported missing.
Prosecutors allege that the fall came after Mr Bankman-Fried stole billions in customer money, allegedly using the funds to buy property, make political donations and fund his crypto trading firm Alameda Research.
Mr Bankman-Fried has denied the charges against him.
Ms Ellison, who headed Alameda Research starting in 2021, is one of the key figures in the case. She pleaded guilty in December to lying to investors and other charges, telling the court: "I knew that it was wrong."
She took the stand shortly after 12:30 local time (16:30 GMT) and told the court that Alameda ultimately took about $14bn (£11.4bn) from FTX customers, using it for investments and repaying lenders.
Alameda was able to return some of that money to the exchange, she added.
"He was the one who set up these systems that allowed Alameda to take the money and he was the one who directed us to take customer money to repay our loans," Ms Ellison said.
She said she repeatedly conferred with Mr Bankman-Fried on the state of the firm's finances, running analyses at his request that showed the firm was risking trouble as it took on loans that could require repayment at any moment.
He allegedly directed her to provide loans to FTX executives and sometimes to buy digital currencies to support their values, even when it led Alameda to lose money.
"For any major decisions, I would always run them by Sam," she said. "He directed me to commit these crimes."
But she would always "defer" to him in decision making, despite her concerns.
She said that at key moments, such as when he was considering setting up a multi-billion dollar fund to invest in new companies, he often moved forward despite her challenges.
He was well aware of the risks, asking her to run analyses of worst case scenarios - many of which later materialised.
But she said he was proudly "risk neutral".
Ms Ellison's arrival on the stand was greeted with nervous laughter, especially after she struggled for a long moment to find Mr Bankman-Fried in the courtroom.
A wide range of people, from former FTX customers to casual observers, has been anticipating her first-hand account of what led the once powerful exchange to collapse last year, turning Ms Ellison into a star witness.
Ben McKenzie, the star of The OC who has written a book critical of crypto, was among the onlookers sitting in the courtroom and watching from overflow locations in the New York courthouse.
She has admitted to a range of charges, including lying about the relationship between the two firms, in expectation of a reduced sentence.
What you need to know about Sam Bankman-Fried trial
Mr Bankman-Fried's lawyers have argued that Alameda's relationship with FTX was characterised by business practices that were reasonable, given its role helping to ease trading on the platform.
In their opening arguments they claimed that the firm's problems were fuelled by Ms Ellison's failure to heed Mr Bankman-Fried's warning to protect the firm against turmoil in the crypto markets.
On the stand on Tuesday, Ms Ellison said she felt she lacked experience to lead Alameda and spoke candidly about her on-and-off again relationship with Mr Bankman-Fried.
"The whole time we were dating he was also my boss," she said, adding that their personal dynamic was also strained. "I wanted more from our relationship but often felt he was distant or not paying much attention to me," she said.
Ms Ellison met Mr Bankman-Fried when they both worked at the investment firm Jane Street.
The daughter of MIT economists who earned a maths degree at Stanford, she shared a family background rooted in academia and an ambition to make money to do good.
She is one of several members of Mr Bankman-Fried's inner circle to take the stand against him.
