Goldman Sachs Group Inc. said it was being investigated for its U.S. alternative trading system and for the potential misuse and circulation of non-public information related to its corporate developments.

Fox Business Network reported this week that New York's attorney general was investigating the alternative trading systems, also known as "dark pools," run by Goldman and Morgan Stanley.

Goldman, in a filing on Thursday, did not specify who was conducting the investigation.

A unit of Goldman Sachs was fined in July over pricing rule violations stemming from its dark pool SIGMA-X.

Dark pools are broker-run trading venues that let investors trade shares anonymously and only make trading data available afterwards, reducing the chance of information leaking about trade orders.

Goldman also lowered its unreserved legal costs to $3.2 billion at the end of June from $3.7 billion at the end of March, it said in the filing.