Recently SEC released a report on the causes of flash crash on May 6, 2010. The report can be found here. They attributed the source of the crash to a single trade made by a mutual fund Waddell & Reed Financial of Kansas (not named in the report). The news can be read here.
Now, Nanex analysis suggests that this does not provide us with a complete picture and W&R most likely did not cause the crash. Interesting read overall!