On Tuesday the Financial Industry Regulatory Authority announced that it has launched a probe into how retail brokers treat customer orders. The aim is to ensure that the brokers are complying with regulations requiring best execution for investor stock orders, The Wall Street Journal reports.

The regulator will be sending letters to 10 unnamed firms seeking records and documentation about their order-routing and execution practices. Tom Gira, executive vice president of FINRA’s Market Regulation Department, told Reuters that the 10 firms were selected because of their order flow.