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Citadel Securities is leading the industry’s fight against an effort by exchanges including the New York Stock Exchange and Nasdaq to charge traders to help pay for a new market surveillance system that has already cost nearly $1 billion.

Brokers are calling on the regulator to suspend proposed new billing schemes that would force them to contribute to the costs of the Consolidated Audit Trail (CAT) system, a real-time record of all activity on US stock and options markets that has been likened to a “Hubble telescope” for securities markets.

Exchanges have so far footed the bill for CAT. But unless the U.S. Securities and Exchange Commission takes action in the coming weeks, brokers will start piling up fees in bills sent by exchanges starting Tuesday as they fight to claw back some of the system’s costs they’ve long been promised.

The CAT was conceived after the flash crash of 2010, when researchers struggled to figure out what caused a market decline that at one point wiped nearly $1 trillion off the value of U.S. securities. The CAT has been fully operational since 2022.

The SEC tasked national exchanges and Finra, which oversees brokers, with building the system, with the understanding that the trading industry would ultimately pay a significant portion of the costs.

The regulator approved a plan last year that would have broker-dealers bear two-thirds of the fees and trade the rest. The first payment plans from the exchanges and Finra were filed in January. Normally, such filings take effect immediately unless the SEC suspends them, which it did in this case pending review. That review is not yet complete.

Last month, the exchanges and Finra withdrew the payment requests and submitted new ones with minor changes. Now, unless the SEC issues another suspension, brokers will receive bills in October based on September trading volumes.

In recent weeks, industry groups including Citadel Securities, Virtu Financial, the American Securities Association and Sifma have filed a flurry of letters and documents with regulators urging them to suspend the proposed legislation.

Citadel Securities, which is controlled by billionaire Ken Griffin, has warned the SEC that it “will have no choice but to take appropriate legal action” if the agency does not suspend the billing by next week.

In a letter last week, the latest documents were described as “brazen” and “an attempt to extort hundreds of millions of dollars from brokerages like Citadel Securities.”

The company, one of the largest market makers in the US stock market, last year challenged the SEC in a Florida court over the legality of the CAT financing model. That case, brought in conjunction with the ASA, is ongoing.

Exchange groups including the New York Stock Exchange, Nasdaq and Cboe Global Markets, which dominate the market, declined to comment. Finra also declined to comment. The SEC did not respond to a request for comment.

Exchange officials privately pointed out that they were tasked with setting up the system by the SEC and that the costs were always intended to be shared with the industry. Since its implementation, CAT’s costs have continued to rise, partly due to rising trading volumes.

“We’re just recouping our costs. There’s no profit here,” said one executive who has been involved with the project for years. “They did everything they could to avoid paying for the CAT. That’s why we’re at $1 billion and the industry is at zero.”

Other concerns raised by real estate agents include whether they should be held liable for costly missteps during the CAT’s construction. They question its annual operating budget, which now stands at nearly $200 million a year, about five times original estimates in 2016.

Brokers have pointed out the large differences between the model invoices drawn up by the committee that manages CAT and their own internal estimates of their contributions. They want to use these estimates as a basis for passing on costs to clients.

Beyond the costs, the CAT has also raised broader concerns about the access it will give regulators to individuals’ trading data. In a separate lawsuit filed in Texas in April, civil rights groups are seeking to have the CAT declared unconstitutional because of what they see as the privacy risk in the personal data it collects.