By day, Navinder Singh Sarao was a determined futures trader — headphones on, hood pulled low, feet on his desk — who boasted market powers that made him a legend among other small-time investors at an electronic-trading facility on the outskirts of London.

At night, Sarao, 36 years old, returned to the modest, semidetached home he shared with his parents, near a flight path to Heathrow Airport. Two brothers lived nearby.