According to the New York Times could mean that the computers first flooded the market with sell orders that could not be matched with buyers. Just as quickly, many of these networks  from trading. The combined effect may have set off a chain reaction that sent shares awry during the 15-minute frenzy.

Specialists at the major exchanges no longer believe that a single large sell trade in one stock, was the trigger. They suspect a mismatch in rules between the older NYSE and younger electronic exchanges set off the frightening sequence of events.

Investigators say the earliest sign of trouble was a sudden drop in value of a futures contract on the Chicago Mercantile Exchange, based on the Standard & Poor’s 500-stock index. That pushed down a broad array of stocks in that index, all of them traded on the New York Exchange and the other major exchanges, sending many stocks on the New York Exchange into slow mode.

Ever since computerised trading became dominant in the stock markets, experts have been warning that lack of consistent rules among exchanges, increasing complexity and the speed of computer trading systems could destabilise markets as appears to have happened last week, when stock prices plunged and the Dow Jones industrial average fell roughly 600 points in a few minutes.

Officials of the Securities and Exchange Commission and the heads of the four main exchanges are to meet to discuss applying circuit breakers across all exchanges. Only the New York Exchange applies circuit breakers on individual stocks. Investigators say the rule on halting trading was created for a time when one exchange accounted for the vast proportion of stock trading.

Over the last half decade the Big Board’s share of the market has dropped sharply, partly because of regulatory changes to encourage new competitors and ever larger volumes of stocks being traded on electronic exchanges without circuit breaker rules.

The switch to slow-motion trading as traders on the floor tried to arrest the decline by manually seeking out bidders did not work, because trading shifted immediately to broader markets controlled by computers, where the plunge continued.

Regulators and the exchanges continued over the weekend to review the tapes from the millions of trades made last Thursday. Investigations are looking at what effect the decision to halt trading in these stocks in New York had on broader market confidence — and on algorithms used by computerised traders.

The scale of the shutdown have been a new phenomenon for these computer systems. They may also have been programmed to shut down in such a cataclysmic moment of stress, which would have had a further cascading effect in withdrawing bidders from the market and putting further intense downward pressure on prices.

Lack of exchanges coordination has been part of the investigation and is considered by regulators to be more of a magnifying event than the actual trigger of the market’s sudden swoon. As trading has been dispersed among a dozen electronic exchanges, the SEC and other market regulators have maintained no centralised database of stock trades, order sizes or prices. That has made it more difficult for regulators to piece together what exactly did happen.

The SEC has been warned in recent months by market participants, publicly traded companies and other regulatory agencies, that lack of coordination between trading platforms as well as expansion of high-speed trading in alternative markets, furthered systemic risk, encouraged regulatory arbitrage and increased opportunities for market manipulation.

The staff of the Financial Industry Regulatory Authority wrote to the SEC in April that “no single regulator has a full picture of all trading activities in the US equity markets.”