The US trading watchdog says May's so-called flash crash, which saw the Dow Jones index fall by nearly 10% within minutes, was caused by a single trader's computer program.
A report by the Securities and Exchange Commission and the US Commodity Futures Trading Commission says a trader used an algorithm to sell 75,000 stocks, worth more than $US4 billion, extremely quickly.
Algorithms are a mathematical process used for calculation and data processing.
The programme had previously executed a similar sale over more than five hours, but on 6 May it was carried out in just 20 minutes.
That caused other super-fast trading algorithms to sell quickly, which caused the Dow Jones index to drop by almost 10%.
Authorities have since introduced circuit breakers, which can halt trades on certain stocks for 10 minutes, if they fall by more than 5%.