In the months before Mt. Gox's collapse, the bitcoin exchange was coming under increasing pressure from one of its banks, which complained about an unmanageable volume of money transfers and repeatedly asked the exchange to close its account with the bank, people familiar with the situation say.

Mizuho Bank, which handled international wire transfers and other Mt. Gox business at a branch in Tokyo's Shibuya district, had become worried about Mt. Gox as early as mid-2013, following reports that the U.S. Department of Homeland...