Data analytics firm Cambridge Analytica is facing allegations of using information from millions of Facebook users to predict and influence the US election.
But what is Cambridge Analytica, who is behind it and what links does it have with Facebook?
US-headquartered Cambridge Analytica uses data to change the behaviour of internet users.
"We map out an in-depth picture of your audiences," it says on website of the firm, which also has offices in the UK.
Cambridge Analytica was hired by Donald Trump's presidential campaign team.
The company allegedly provided data on the thoughts of American voters to Mr Trump's campaign strategists, ultimately influencing the 2016 presidential election.
Cambridge Analytica says it deleted the data when it was asked to.
Aleksandr Kogan is the University of Cambridge professor who developed an app which collected the data.
He developed 'This Is Your Digital Life' through his company Global Science Research (GSR) in collaboration with Cambridge Analytica.
The app offered payment in return for users filling out a personality test.
Facebook says it was downloaded by 270,000 people, but the app also gave Mr Kogan access to the lists of the downloaders' Facebook friends, which, according to Cambridge Analytica whistleblower Christopher Wylie, gave him information on up to 50 million more users.
Facebook suspended Cambridge Analytica when it found out about the data breach, saying that in 2015 it found out that "a psychology professor at the University of Cambridge named Dr Aleksandr Kogan violated our platform policies."
But Mr Kogan says he is being unfairly represented and that when he developed the This Is Your Digital Life app, he thought it would be used legally.
"My view is that I'm being basically used as a scapegoat by both Facebook and Cambridge Analytica," he told BBC radio.
"Honestly, we thought we were acting perfectly appropriately."
Alexander Nix, 42, is the chief executive of Cambridge Analytica.
He developed the political arm of the company which has since been involved in election campaigns around the world, according to The Guardian/Observer.
He has been suspended by Cambridge Analytica's board pending a full independent investigation after he was filmed by undercover Channel 4 reporters talking about ways to win political campaigns through blackmail and honey traps.
Mr Nix's comments "do not represent the values or operations of the firm", the company said.
Christopher Wylie blew the whistle on this scandal.
The 28-year-old Canadian data analytics expert worked with Cambridge Analytica and Cambridge professor Aleksandr Kogan to obtain people's data.
He told The Observer Cambridge Analytica used personal information taken without permission in early 2014 to build a system that could profile US voters so they could be targeted with personalised political advertisements.
Facebook has suspended his account.
Cambridge Analytica says he is a former contractor who left the company in 2014 and has been "misrepresenting himself and the company" in his comments to the media.
Mr Trump's former chief strategist is a founder and former board member of Cambridge Analytica.
Mr Bannon was also founding member of right-wing news and commentary website Breitbart News.
He used the publication to help push an anti-Washington message and was a key figure in Mr Trump's election.
Christopher Wylie told the Washington Post that Mr Bannon gave approval for the firm to spend nearly $1m to acquire data, including Facebook profiles, in 2014.
It is unclear whether Mr Bannon knew how Cambridge Analytica was obtaining the Facebook data.
Robert Mercer and his family part-owns Cambridge Analytica.
Mr Mercer founded the company along with Mr Bannon.
Mr Mercer is a staunch conservative and is said to have invested $15m in Cambridge Analytica, as well as being a very generous Republican donor.
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He used to be an IBM computer scientist before going on to work with computer-driven hedge fund Renaissance Technologies where he became a billionaire.
He quit as CEO in November after being scrutinised by the media for backing Donald Trump and his former strategist Steve Bannon, which included investing heavily in Mr Bannon's Breitbart News.