Representatives Greg Walden, Marsha Blackburn, Gregg Harper and Robert Latta wrote to Apple’s chief executive Tim Cook and Alphabet chief executive Larry Page in July, citing concerns about reports that smartphones could “collect ‘non-triggered’ audio data from users’ conversations near a smartphone in order to hear a ‘trigger’ phrase, such as ‘Okay Google’ or ‘Hey Siri’”.

In a letter to Walden, an Oregon Republican who chairs the House Energy and Commerce Committee, Apple said iPhones do not record audio while listening for Siri wakeup commands and Siri does not share spoken words. Apple said it requires users to explicitly approve microphone access and that apps must display a clear signal that they are listening.

The letters, in which lawmakers cited reports suggesting third-party applications had access to and used ‘non-triggered’ data without users’ knowledge, followed congressional hearings in April into Facebook privacy practices, which included testimony by its CEO Mark Zuckerberg.

Apple wrote that it had removed apps from its App Store over privacy violations but declined to say whether it had ever banned a developer. It also said it was up to developers to notify users when an app was removed for privacy reasons.

“Apple does not and cannot monitor what developers do with the customer data they have collected, or prevent the onward transfer of that data, nor do we have the ability to ensure a developer’s compliance with their own privacy policies or local law”, Apple wrote.

The iPhone maker’s App Store has generated $100 billion in revenue for developers over the past decade. Apple told lawmakers in its letter that it rejected about 36,000 apps from among the 100,000 submitted each week for violations of its guidelines.