The multibillion-dollar data brokerage industry, a growing force in online marketing, is drawing intensified government scrutiny.

On Wednesday, Senator John D. Rockefeller IV, Democrat of West Virginia, opened an extensive investigation of nine leading information brokers. Because Americans now conduct much of their daily business online, the senator said he was concerned that “an unprecedented amount” of personal, medical and financial information about people could be collected, mined and sold, to the potential detriment of consumers.

“An ever-increasing percentage of their lives will be available for download, and the digital footprint they will inevitably leave behind will become more specific and potentially damaging, if used improperly,” Mr. Rockefeller, who is the chairman of the Senate Committee on Commerce, Science and Transportation, wrote in letters to the data brokers. “It is critical that we understand what information companies like yours are already collecting and selling.”

Linda A. Woolley, the acting chief executive of the Direct Marketing Association, a trade group, called the senator’s investigation “a baseless fishing expedition.”