From a modest stucco house in suburban west London, where jetliners roar overhead on their approach to Heathrow Airport, a small-time trader was about to play a hand in one of the most harrowing moments in Wall Street history.Navinder Singh Sarao was as anonymous as they come—little more than a day trader by the standards of the Street.But on that spring day five years ago, US authorities now say, Sarao helped send the Dow Jones Industrial Average on the wild, 1,000-point ride that the world came to know as the flash crash. By regulators’ account, he was responsible for a stunning one out of five sell orders during the frenzy. On Tuesday, he was arrested by Scotland Yard and charged in the US with 22 criminal counts, including fraud and market manipulation. The news left many grasping for answers. Sarao, 36, has no record of having worked at a major financial firm in the US or the UK. At the time of the flash crash, Sarao was renting space from a proprietary trading firm in the City of London and clearing his transactions through MF Global Holdings, the now-defunct firm headed by Jon Corzine, said a person with knowledge of the matter. One of Sarao’s neighbors in Hounslow, 11 miles from central London, said what neighbours so often say: He was quiet, kept to himself, never caused trouble.That picture, according to US authorities, belies a years-long history of lightning-quick computer trading that netted Sarao $40 million in illicit profits. Sarao couldn’t be reached for comment on Tuesday and US authorities said they didn’t know whether he had retained a lawyer. Sarao didn’t cause the flash crash single-handedly, authorities say. Nonetheless, Tuesday’s developments fly in the face of the prevailing narratives of what happened. Regulators initially concluded that a mutual fund company— said to be Waddell & Reed Financial of Overland Park, Kansas — played a leading role. Many in the industry countered that a confluence of several forces, including high-frequency trading, was probably behind the crash.By all accounts, the flash crash was more than a mere technical glitch. It raised fundamental questions about how vulnerable today’s complex financial markets are to the high-speed, computer-driven trading that has come to dominate the marketplace.Little is known about Sarao and his trades, beyond what is contained in a complaint filed by the US Department of Justice. A related civil suit filed by the US Commodity Futures Trading Commission provides a few additional glimpses into his supposed activities. The case stemmed from a whistle-blower who brought “powerful, original analysis” to the CFTC’s attention, said Shayne Stevenson, a Seattle lawyer representing the whistle-blower.According to US authorities, Sarao spent the past six years thumbing his nose at regulators while using software designed to manipulate markets. In addition to fraud and manipulation, he was charged with spoofing — an illegal practice that involves placing orders with the intent to cancel before they’re executed. In May 2010, Sarao’s actions created imbalances in the derivatives market that then spilled over to stock markets, exacerbating the flash crash, according to the CFTC.“We do believe and intend to show that his conduct was at least significantly responsible for the order imbalance that in turn was one of the conditions that led to the flash crash,” Aitan Goelman, the CFTC’s director of enforcement, told reporters on Tuesday.When he was trading, Sarao kept to himself. His computer screen almost always flashed futures data tied to the Standard & Poor’s 500 Index and his interactions were typically limited to workers installing new trading algorithms, said the person, who spoke on the condition of anonymity.When he started his allegedly manipulative trading in 2009, Sarao used off-theshelf software that he later asked to be modified so he could rapidly place and cancel orders automatically. At one point, he asked the software developer for the code, explaining that he wanted to play around with creating new versions, according to regulators.