The Financial Industry Regulatory Authority has launched a probe into how retail brokers treat customer orders amid mounting concerns some brokers might be sending orders to venues that provide the highest payments but not the best price for investors.

Finra this week is sending "sweep letters" to about 10 brokers seeking information about how they route customer orders, Finra officials said. The regulator decided to press brokers for details about how they route customer orders after studying routing patterns by the firms on its Order Audit Trail System, which tracks stock market trading action, said Tom Gira, executive vice president of Finra's market regulation department, in an interview.