Securities regulators proposed improving market surveillance on Wednesday by tracking stock orders across all U.S. equity markets in real time.

The measure, under development at the U.S. Securities and Exchange Commission for months, would likely have helped the agency piece together the brief May 6 crash in stock prices.

It is shocking that the SEC does not have its own direct access to market data, SEC Commissioner Luis Aguilar said at a public agency meeting. Most Americans assume that the SEC already has these tools and is constantly monitoring the market.

Hindered by their inability to easily see the entire marketplace, the SEC and other regulators are still analyzing the market swoon that saw the Dow Jones industrial average plunge some 700 points in minutes before recovering.

SEC Chairman Mary Schapiro said analyzing the events of May 6 has been substantially more challenging and time consuming because no standardized, automated system exists to collect data across the various trading venues, products and market participants.

The consolidated audit trail, proposed unanimously by the five SEC commissioners, would allow us to rapidly reconstruct trading activity and to quickly analyze both suspicious trading behavior and unusual market events, Schapiro said at the same meeting.

But even if the SEC adopted the plan on Wednesday, it would take at least three years for it to be fully operational because of the complexity involved in implementing new systems and standardizing information across the dozens of trading venues.

Market surveillance is currently shared by in-house teams at trading venues like the New York Stock Exchange and by the Financial Industry Regulatory Authority (FINRA), an industry-backed broker-dealer watchdog.

Under the proposal, trade information for every stock and listed option would have to be reported to a central repository in real time. That repository would be owned and operated by FINRA and the major U.S. exchanges including the New York Stock Exchange, Nasdaq and BATS Global Markets.

The SEC estimates it would cost the trading industry $4 billion to implement and $2.1 billion in annual maintenance.

The central repository would consolidate and retain all the information on quotes, orders and trades for stocks and listed options. It could eventually be expanded to include futures markets and products and over-the-counter derivatives.

For nearly three weeks, regulators have been analyzing more than 19 billion shares of stock that were traded on May 6. They still have been unable to pinpoint the cause of the free fall.

Regulators have had to request the trading data from a host of market players such as the exchanges and broker-dealers.

The SEC proposal will be open for public comment for 60 days. The SEC must then vote to adopt the plan before exchanges and FINRA can start creating the consolidated audit trail and central repository.

(Reporting by Rachelle Younglai; Editing by Tim Dobbyn, Leslie Gevirtz)