A group of lawmakers say data-mining companies that collect and sell personal information about consumers should make their operations more transparent.

Members of the Congressional Bipartisan Privacy Caucus said Thursday that responses they received from nine major data brokers provided only a glimpse of a multibillion-dollar industry "that has operated in the shadows for years." Data brokers tap a variety of sources for consumer information, including mobile phones and social media sites such as Facebook and LinkedIn. The data is then packaged and sold to advertisers and retailers seeking to tailor their marketing campaigns to specific customers.

The privacy caucus sent letters to the data brokers in July. One of the main questions it wants answered is how data brokers rate consumers and group them into categories.