New York state's attorney general is suing British banking and financial services firm Barclays.
Attorney General Eric Schneiderman announced the securities fraud lawsuit today.
He accuses the firm of making false statements about its private stock trading platform and other aspects of its electronic trading division.
He is seeking unspecified damages and wants Barclays to change what it is doing.
The lawsuit alleges Barclays told investors it had set up safeguards to protect them from predatory high-frequency traders. But it says Barclays actually catered to those traders.
The private stock trading platform is known as a dark pool. Mr Schneiderman says Barclays' dark pool was full of predators "who were there at Barclays invitation".
Barclays PLC has no comment on the lawsuit.