The Federal Trade Commission on Tuesday opened an investigation into data brokers, the companies that collect and sell information about consumers for marketing and other purposes.

The agency issued administrative subpoenas to nine information resellers, requiring each company to provide extensive details about how it collects information about consumers; how it uses, stores, analyzes and shares that data; and whether the company allows consumers to access and correct the records the company holds about them. Some of the companies named in the inquiry do not directly sell information about consumers, but offer analytics services in which they categorize, score or evaluate consumer data.

The companies include Acxiom of Little Rock, Ark., one of the world's largest information resellers, which manages customer databases for major banks, automakers and retailers; eBureau, a company in St. Cloud, Minn., which, on behalf of clients like credit card companies, lenders, insurers and educational institutions, evaluates and scores online consumers in the market for those companies' products; Intelius, a company in Bellevue, Wash., which offers people-search look-up services and background checks; and PeekYou, a company that analyzes social media sentiment. Acxiom and eBureau were the subjects of separate articles this year in The New York Times.

In an email message in response to a reporter's query, Gordy Meyer, the president of eBureau, wrote that the company "welcomes the opportunity to describe, to the FTC, its practices and the benefits we provide to businesses as well as consumers."

Representatives of other companies did not immediately return emails from a reporter seeking comment.

The FTC's action comes nine months after the agency issued a report on consumer privacy, calling on data brokers to make their practices more transparent to the public.

Because most data brokers are business-to-business enterprises, regulators say, many consumers are not aware that such companies may compile and sell hundreds of details about their race or ethnicity, financial status, shopping habits, health interests, vacation preferences, Web browsing history, online search queries and other matters.

"Data brokers aggregate huge amounts of data on individuals and have the capacity to create powerful profiles combining information about what you do offline and online," David C. Vladeck, the director of the FTC's Bureau of Consumer Protection, said in a phone interview Tuesday. "We worry that this information may be used in ways that could be harmful to consumers."