"You still have everyone's bitcoin?" bitcoin trader Kolin Burges asked Mark Karpeles, the CEO of Japan-based company Tibanne, which operates bitcoin exchange Mt.Gox, on Friday morning.

Unhappy about information coming out of Mt.Gox about recent withdrawal problems, Mr. Burges, a London resident, this week bought a last-minute ticket to Tokyo for 850 pounds (around $1,400) to get better answers. He arrived in Japan on Wednesday and requested an appointment with Tibanne executives. He received no reply, he said, and...