The FBI is investigating whether high-frequency trading companies carry out insider trading or fraud by abusing fast-moving information they receive before other investors.
The agency is also looking at whether traders use the information to act ahead of orders by other investors, and whether they're manipulating the market by placing a set of trades and then very quickly canceling them.
News of the probe, which has been going on for a year, comes after author Michael Lewis accused high-speed trading firms of rigging the market in a new book called "Flash Boys."
The investigation adds to inquiries by other authorities, including New York Attorney General Eric Schneiderman. The Commodity Futures Trading Commission and the SEC are reportedly looking at the matter as well.