Whistleblower describes how firm linked to former Trump adviser Steve Bannon compiled user data to target American voters

The data analytics firm that worked with Donald Trump’s election team and the winning Brexit campaign harvested millions of Facebook profiles of US voters, in one of the tech giant’s biggest ever data breaches, and used them to build a powerful software program to predict and influence choices at the ballot box.

A whistleblower has revealed to the Observer how Cambridge Analytica – a company owned by the hedge fund billionaire Robert Mercer, and headed at the time by Trump’s key adviser Steve Bannon – used personal information taken without authorisation in early 2014 to build a system that could profile individual US voters, in order to target them with personalised political advertisements.

Christopher Wylie, who worked with a Cambridge University academic to obtain the data, told the Observer: “We exploited Facebook to harvest millions of people’s profiles. And built models to exploit what we knew about them and target their inner demons. That was the basis the entire company was built on.”

Documents seen by the Observer, and confirmed by a Facebook statement, show that by late 2015 the company had found out that information had been harvested on an unprecedented scale. However, at the time it failed to alert users and took only limited steps to recover and secure the private information of more than 50 million individuals.

The New York Times is reporting that copies of the data harvested for Cambridge Analytica could still be found online; its reporting team had viewed some of the raw data.

The data was collected through an app called thisisyourdigitallife, built by academic Aleksandr Kogan, separately from his work at Cambridge University. Through his company Global Science Research (GSR), in collaboration with Cambridge Analytica, hundreds of thousands of users were paid to take a personality test and agreed to have their data collected for academic use.