SEC summons exchange chiefs in wake of Dow plunge

The watchdog is still poring over data to determine the cause of last week's massive sell off but in the meantime has called the heads of Nyse Euronext, Nasdaq OMX, Direct Edge and Bats for discussions on ways to prevent a repeat.

The meeting will consider introducing a market-wide stock-specific circuit breaker to slow down frantic computer driven trading.

During Thursday's tailspin, Nyse Euronext halted electronic trading in some stocks in a move designed to force orders over to floor traders. However, with other, all-electronic, platforms not employing similar circuit breakers, order flow was simply redirected.

Nasdaq OMX CEO Robert Greifeld told CNBC that Nyse Euronext's move had compounded the problem by sending a signal that there was something wrong with the stocks. Nyse Euronext chief Duncan Niederauer hit back, saying the mechanism was to slow down "the race car when we think it's dangerous".

The pair have since issued a joint statement stressing their determination to work together and with the SEC to "develop effective solutions promoting greater market stability, efficiency and transparency".

Today they are meeting with other exchange bosses and the SEC to discuss bringing their procedures, rules and regulations more closely into line in a bid to prevent a rerun.

Meanwhile, the hunt for a cause continues. A fat finger trade has now been ruled out as a potential trigger for the meltdown, according to Reuters while the Homeland Security Department says there is no evidence that a computer attack was responsible.

A theory still not dismissed is that a huge computerised trading program was triggered by a big currency-related play amid Greece crisis, says Reuters.

UpdateThe SEC issued a statement later in the day: "As a first step, the parties agreed on a structural framework, to be refined over the next day, for strengthening circuit breakers and handling erroneous trades."