“Regulators are ramping up scrutiny of an opaque corner of the market where stocks change hands in the dark.

The Financial Industry Regulatory Authority, Wall Street’s self-regulatory body, last month sent 15 examination letters to operators of “dark pools”—lightly regulated, off-exchange trading venues that have been a rising concern for regulators and some investors as more activity shifts away from exchanges.

Finra is seeking details about how the increasingly popular venues operate, what they disclose to clients and whether they adequately police trades. It could bring enforcement action against dark-pool operators or issue recommendations for tighter oversight, depending on the answers it receives and additional examinations, said John Malitzis, executive vice president of market regulation at Finra. The letters are a follow-up to an initial round of questions the regulator circulated last fall.

“We want to understand whether [dark pools] are disclosing to their customers how their orders work [and] whether customers are informed who their orders will interact with,” Mr. Malitzis said in an interview. “A big part of this is to get an understanding of practices that may or may not be problematic.”

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