For a very long time, there were rumors about the cause of this event. From fat finger on Procter & Gamble, impact of high frequency traders, large directional bets or technical glitches. Also fundamental background was very "hot" - In the UK it was general election day, in the US,Wall Street was gripped by mounting anxiety about the Greek debt crisis. The euro was falling against the dollar and the yen. After five years has been charged a high-frequency futures trader for illegal manipulating with stock market.