If the FTC finds that the companies do not change their business practices, or if complaints are filed, the FTC could launch a formal investigation or eventually levy fines against the companies.
While most of the 10 companies promised general background checks, some of the companies identified by the FTC on Tuesday offered other more personalized data.
A company called Brokers Data, based in Columbia, S.C., for example, provided information that could be used to determine a person's eligibility for insurance. And in the case of ConsumerBase, the FTC said the Chicago-based company appeared to be selling lists of consumers who meet a certain financial criteria to lenders who make pre-approved credit offers.
In its letter, the FTC warned ConsumerBase that its sale of such lists could violate the law if ConsumerBase wasn't making sure the lists were accurate or notifying consumers.
"We want to make you aware of the requirements of this law so that you can ensure that your practices comply with it, insofar as you engage in such sales," the FTC wrote.
Brokers Data, ConsumerBase, 4Nannies.com and USA People Search did not immediately return phone calls requesting comment.