Sam Bankman-Fried’s trading firm Alameda Research reportedly owes his crypto exchange FTX $10 billion after taking loans funded by deposits from FTX customers, according to The Wall Street Journal.

As economist Frances Coppola pointed out to the Journal, exchanges like FTX shouldn’t be investing customers’ money. “It shouldn’t be doing anything with those assets. They should literally be sitting there so people can use them,” said Coppola. That’s especially true for volatile markets like crypto, where collateral could swing in value from one day to the next.

However, FTX reportedly lent over half of its customer funds to Alameda, which then used them to bet on other cryptocurrencies and help out other crypto firms struggling to weather an overall downturn in the market.

FTX’s troubles started with a CoinDesk article that called Alameda’s balance sheet into question by saying that much of it was made up of FTT, the exchange’s token. That turned into a real problem when Binance founder Changpeng Zhao announced plans to sell the billions of FTT it held, which sent the value of the cryptocurrency into an unrecoverable tailspin. FTX has since struggled to stay afloat, with its customers trying to withdraw funds amid fears over just this kind of risky financial arrangement between FTX and Alameda. Data trackers like Nansen have seen some successful withdrawals processed by FTX on Thursday, but who has been able to extract funds and why is still unclear.

Both Alameda and FTX are controlled by Sam Bankman-Fried, who issued a public apology this morning, saying that he “fucked up, and should have done better.” And while he didn’t directly address whether Alameda had been using FTX customer funds, he did say that the firm is “winding down trading” and claimed it isn’t “doing any of the weird things that I see on Twitter.”

The future for FTX is up in the air at this point, though things aren’t looking great. The largest crypto exchange, Binance, announced earlier this week that it intended to buy the firm to ensure customers got their funds back. It promptly took back that offer the next day, claiming its due diligence unearthed “issues are beyond our control or ability to help.” Bankman-Fried is reportedly trying to raise $9.4 billion from various investors for an FTX rescue package, according to Reuters. In his Twitter thread, Bankman-Fried said “every penny” of liquidity raised would go to customers until the firm had “done right by them.”

Bankman-Fried also promised that FTX.us users are fine (the domestic exchange operates as a separate entity) and that everything going on relates to the FTX.com exchange available internationally. However, the FTX.us site now has a warning saying that “trading may be halted on FTX US in a few days” and tells investors to “close down any positions you want to close down.” It still says that “withdrawals are and will remain open.”