Early last summer, Adam Yedidia took Sam Bankman-Fried to one side after a game of paddle tennis. In the shadow of a hut in the grounds of the Bahamian penthouse they and others shared, he asked the crypto tycoon: “Are we OK?”

Yedidia told a New York court this month that he was worried FTX, the crypto exchange that Bankman-Fried had co-founded and at that time led, was in financial trouble. He recounted his old friend’s reply: “We were bulletproof last year, but we’re not bulletproof this year.”

FTX collapsed a few months later, sending shockwaves through the cryptocurrency industry. US prosecutors hope this chat, which they have taken to calling the “bulletproof conversation,” will help to show Bankman-Fried knew about, and concealed, an $8 billion cash shortfall for months at least before it was exposed.