An industry-funded regulator Wednesday agreed to take on oversight of two more U.S. exchanges, a move that will improve its surveillance and expand its coverage to around 90% of daily stock trading.

The head of the Financial Industry Regulatory Authority said the new arrangement with exchange operator Direct Edge Holdings LLC will leave Finra better positioned to sniff out abusive trading schemes that stretch across multiple trading platforms.

“We’re in a position now to look across markets in a way that simply wasn’t possible a few years ago,” Richard Ketchum, Finra’s chief executive, said in an interview. Direct Edge accounts for about 12% of U.S. stock trading.

Finra is the main watchdog for manipulative trading and other illegal behavior on U.S. stock markets. The federal Securities and Exchange Commission, which regulates exchanges, relies on such self-regulatory organizations to keep tabs on brokerages and examine abusive market practices on exchanges and private trading platforms such as “dark pools.”

The body traces its roots to the National Association of Securities Dealers, reforming as Finra in 2007 when it consolidated oversight of firms doing business on the Nasdaq Stock Market and New York Stock Exchange. It expanded by taking on surveillance of trading, in 2010 assuming oversight of stock and options markets run by NYSE Euronext (NYX).

Finra already examined and disciplined firms that are members of Direct Edge’s two electronic stock exchanges. The new agreement lets Finra fold the two platforms into the agency’s cross-market surveillance system developed after assuming watch over NYSE markets, Mr. Ketchum said.

The expanded system, rolled out last August, is made up of various “patterns” that monitor markets for about 50 separate scenarios that could indicate market malfeasance. Mr. Ketchum said Finra is now pursuing “a number of reviews” triggered by the new system, which may result in sanctions or penalties.

He said some of these reviews focus on trading that appears to inflate or depress share prices artificially at the end of a trading session–a type of manipulation known as “marking the close.” They also encompass strategies that place trades for the purpose of misleading other traders into driving a bigger move in a stock’s price, a practice called “momentum ignition.”

“Most of what we see here is not rocket science; it’s just done at higher speeds now,” said Mr. Ketchum. The challenge is detecting enough of the activity to trigger a deeper review, and having Direct Edge’s markets within Finra’s surveillance program will make this easier, he said.

“These cops need data,” William O’Brien, chief executive of Direct Edge, said in an interview. He said the decision to give Finra supervisory responsibilities over his exchanges was partly driven by seeing Finra’s tougher surveillance measures in action.

The move by Finra to assume essentially all of Direct Edge’s third-party regulatory services comes while exchanges, banks and regulators debate the legal standing of exchanges as market regulators, particularly as so many functions have been absorbed by Finra in recent years. Finra doesn’t operate any exchanges or trading platforms.

SEC commissioners Daniel Gallagher and Luis Aguilar have raised concerns about the murky status of exchanges, which are largely for-profit enterprises that also bear responsibility for regulating their customers. The Chicago Board Options Exchange, which competes with Finra to provide regulatory services for stock and options markets, has been under investigation for more than a year by the SEC over the exchange’s duties as a self-regulatory organization.

In a speech in Washington earlier this month, Mr. Aguilar highlighted “inherent conflicts of interests” in exchanges overseeing their own members. “To stay in business, SROs have to attract order flows, and this may lead to SROs being less inclined to enforce rules vigorously against financially supportive members, issuers and shareholders,” he said.

Write to Jacob Bunge at jacob.bunge@dowjones.com

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