Fast Traders, in Spotlight, Battle Rules

As a group, they earned $12.9 billion in profit in the last two years, according to the Tabb Group, a specialist on the markets. Tabb expects their earnings to slow this year as Wall Street's big brokerage firms fight back with their own faster computerized trading.

The S.E.C. started to think these firms needed tighter controls in early 2009 when analysts for the first time began to point to the sector's billions in profit, and critics wondered whether their technological firepower gave them an unfair advantage.

The scrutiny intensified after the May 6, 2010, flash crash, one of the most abrupt market moves in recent history, when stocks plunged some 700 points in minutes before recovering.