FBI launches probe into high-speed stock trades

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The last decade has seen the stock market increasingly influenced by high frequency trading -- thousands of computers on trading floors that are able to match buyers and sellers in fractions of a second. Firms make money on the "spread," or difference between buy and sell price. 

But is HFT kosher? 

The practice has gained notoriety recently, as the FBI launched an investigation into the practice, and well-known author Michael Lewis released a book on the subject. 

Detractors say HFT allows large firms an unfair trading advantage. They are able to see trades coming in ahead of time, and sell shares back to "average" investors at a higher price. 

Others have argued HFT is actually beneficial to everyday investors because it forces big firms to compete for their services.