The companies said other sources of personal data include telephone directories, mobile phones, government agencies and financial institutions.
The lawmakers said they weren't satisfied with the answers and that many questions remain about the practices of data brokers.
“The data brokers’ responses offer only a glimpse of the practices of an industry that has operated in the shadows for years,” the lawmakers said in a joint statement. “Many questions about how these data brokers operate have been left unanswered, particularly how they analyze personal information to categorize and rate consumers."
The lawmakers warned that the industry collects information on nearly all Americans, including children and teens.
"We want to work with the data broker industry so that it is more open about how it collects, uses, and sells Americans’ information. Until then, we will continue our efforts to learn more about this industry and will push for whatever steps are necessary to make sure Americans know how this industry operates and are granted control over their own information,” they said.
The letters were sent by Reps. Edward MarkeyEd MarkeyGreens slam Trump’s Interior Department pick Senate sends annual defense bill to Obama's desk Overnight Cybersecurity: Fed agency IT report cards | Senate Dems push for briefing on Russia hacks MORE (D-Mass.), Joe Barton (R-Texas), Henry Waxman (D-Calif.), Steve Chabot (R-Ohio), G.K. ButterfieldG.K. ButterfieldJustices consider redistricting in Virginia, NC La. rep picked to lead Congressional Black Caucus Black Caucus head won't commit in Dems' leadership race MORE (D-N.C.), Austin Scott (R-Ga.), Bobby Rush (D-Ill.) and Jan Schakowsky (D-Ill.).
All of the companies except for Acxiom rejected the label of "data broker." One company called itself a "data provider" and another said it only "analyzes" data.
Acxiom was also the only company that provided information about how many consumers request access to their information. The company said that over the last two years as few as 77 people requested access to their personal information. The company collected information on 190 million consumers in that time period.