Goldman Sachs disclosed Thursday that regulators have questioned it over its so-called "dark pool" in the latest sign of increased concern over the unregulated securities trading platforms.
The giant investment bank added its "US alternative trading system" to a list of operations that are under regulatory review or investigation, according to a quarterly securities filing.
The filing gave no other details on the nature of the probe.
Regulators, including US Securities and Exchange Commission Chair Mary Jo White, have said more oversight is needed for trading venues like dark pools, which have captured an increased share of overall share trading in recent years.
Dark pools allow clients to trade large volumes of shares anonymously with prices posted only after the transaction is finished, avoiding the open reporting of ongoing bid and offer prices required on public exchanges.
New York Attorney General Eric Schneiderman in June filed suit against Barclays, alleging it defrauded investors in its dark pool by operating the platform to the benefit of high-frequency traders.
UBS and Deutsche Bank have also disclosed regulatory inquiries into their dark pools.
Goldman also trimmed its estimate for total litigation costs it has not reserved for from $3.7 billion in March to $3.2 billion.