He operated from a modest suburban London home he shared with his parents, far from the city's glamorous financial center. He used off-the-shelf software anyone can buy.
Yet, if U.S. authorities are correct, Navinder Singh Sarao, 36, managed to send a jolt of fear through the world's markets by helping to set off the 2010 "flash crash," in which the Dow Jones average plunged 600 points in less than seven minutes.
Just how big a role he played has been hotly debated since the federal complaint was unsealed earlier this week, but the idea that a little-known investor had even a small part is deeply troubling, say traders and market experts.
"If this guy...