The 2010 Flash Crash—where the market lost and then regained almost 1,000 points within the course of only a few minutes—rattled Wall Street when it happened, and led to increased attention on the use of algorithmic trading programs that, at this point, make up the majority of securities trades. The SEC and CFTC studied the incident and, in its later report, said the crash came from the confluence of a number of unfavorable factors such as an already jittery market nervous about the Greek credit crisis, an unusually large sell by an institutional mutual fund, and high-frequency trading bots that reacted to both. 

The U.S. Justice Department believes there was one more that can be added to the list: a single trader in the U.K. whom the government is currently trying to extradite to face charges here, according to Bloomberg. The trader, Navinder Singh Sarao, is believed to have been a "significant factor" in the 2010 crash. 

A separate Bloomberg article described him as little more than a day-trader who was none the less responsible for 20 percent of sell orders during the crash. Much of the market manipulations he is accused of centers around an activity called "spoofing," where someone fills an order but then cancels it at the last minute in order to move the price (in this case by giving a false impression of higher supply, thus lowering prices.) Sarao's program, according to the Justice Department complaint, would move price offers up or down depending on the actions of other market participants, but always in a way that ensured those orders would never get filled and would later get cancelled, thus keeping his downward pressure on the market as a whole constant.

The Justice Department believes this activity became a major factor in the 2010 Flash Crash. The complaint said that the steady sell-side pressure Sarao had exerted on the E-Mini market began leaking into the equities market, which then sparked off the famous catastrophic decline. 

Currently under house arrest in the U.K., he is fighting extradition to the U.S. The extradition hearing is tentatively set for August, said Bloomberg.