San Francisco, June 25 : Four Democratic lawmakers have called on the US Federal Trade Commission (FTC) to investigate tech giants Apple and Google over selling personal data. According to The Wall Street Journal, the lawmakers alleged the companies engaged in unfair and deceptive practices by enabling the collection and sale of mobile-phone users' personal information. Apple and Google "knowingly facilitated these harmful practices by building advertising-specific tracking IDs into their mobile operating systems", the lawmakers wrote in a letter to FTC chair Lina Khan. Both companies have recently taken steps to limit the collection of user data through these mobile-ad identifiers, a string of numbers and letters built into iOS and Android. Users of both operating systems now have a way to opt-out of having their identifier transmitted to apps. Apple last year introduced a new version of its software that requires each app to ask the user for permission to access the device's identifier, and Google is planning to adopt new privacy restrictions to curtail tracking across apps on Android smartphones. A Google spokesman said the company never sells user data and said its app store, Google Play, prohibits the sale of data by developers. Meanwhile, Apple did not respond to requests for comment, and the FTC declined to comment /IANS