TOKYO—Scared, frustrated and angry—that is how Mark Karpelès, head of defunct bitcoin exchange Mt. Gox, said he felt when he realized in February that the exchange had lost nearly half a billion dollars' worth of the internet currency.

He said he has spent many sleepless nights since then. "I was always worried: 'What if all the bitcoins got stolen?' " the 29-year-old Frenchman said in an interview, his first since a Feb. 28 news conference to announce the exchange's bankruptcy filing. The Tokyo-based exchange blamed...