BitMEX was hit with a proposed class action seeking the return of 622.66 Bitcoin, worth roughly $40.7 million, on the same day the crypto derivatives exchange announced plans to shut down in September.

BKX Services and trader David Namdar filed the complaint Thursday in the US District Court for the Southern District of New York. BKX claims it lost at least 305.81 BTC through forced liquidations, while Namdar alleges losses exceeding 316.85 BTC.

The complaint alleges BitMEX designed its liquidation system to retain customer collateral and transfer remaining Bitcoin into its insurance fund. It also claims an internal trading desk accessed private customer information and continued operating during server outages that prevented users from managing or closing their positions.

The plaintiffs are seeking the return of the allegedly withheld Bitcoin, along with compensatory and punitive damages. They are asking to represent US customers who traded BitMEX Bitcoin swap products in transactions dating back to July 23, 2018.

BitMEX rejected the allegations, calling the case an opportunistic claim without basis and saying it plans to defend itself.

The complaint was filed as BitMEX said it would close its exchange on September 23 at 4:00 UTC following a strategic review by parent company HDR Global Trading. The platform has stopped accepting new registrations and urged customers to close positions and withdraw their assets before the closure.

BitMEX launched in 2014 and became one of the crypto industry’s most prominent derivatives platforms after popularizing the perpetual swap. Its market share has since fallen below 0.01%, with daily trading volume of about $400,000, according to Kaiko data cited by Reuters.