Whistle-blower Christopher Wylie told a UK parliamentary committee there could have been a different Brexit referendum outcome had it not been for “cheating” by proponents wanting to leave the European Union.

Wylie’s hearing on Tuesday was the latest in a flurry of news events since a story broke on how a company called Cambridge Analytica allegedly used illegally obtained data of more than 50 million Facebook users in an attempt to influence political outcomes.

Here’s a breakdown of the fast-developing story so far:

What’s the controversy?

On March 17, the London Observer and The New York Times reported that UK-based data firm Cambridge Analytica acquired millions of Facebook users’ personal information to build software that could target potential swing voters in political campaigns, including US President Donald Trump‘s 2016 election bid.

The newspapers broke the story with the help of Cambridge Analytica’s cofounder and now whistle-blower Christopher Wylie.

“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,” he told the Observer.

The data his firm used was reportedly gathered in early 2014 through an app called “thisisyourdigitallife”, which was built by a Russian-American researcher at Cambridge University called Aleksandr Kogan.

About 270,000 users agreed to have their data collected and used for academic research in exchange for a small payment. The Times reported Cambridge Analytica covered the costs of the app, more than $800,000.

However, the app not only collected personal information from the people who downloaded it, but also of unknowing friends.

Kogan provided information from more than 50 million Facebook users to Cambridge Analytica, which was used by the company to create 30 million “psychographic” profiles that could then be used to design targeted political ads.

What happened next?

The initial reports set in motion a string of allegations and events that have been difficult to keep up with.

Investigations were launched on both sides of the Atlantic. In the US, the Federal Trade Commission confirmed on Monday it was investigating Facebook’s privacy practices. UK authorities raided Cambridge Analytica’s London offices last Friday night.

Cambridge Analytica said on March 17 it deleted all data received from Kogan’s company, Global Science Research (GSR), when it found out it had not been obtained in line with Facebook’s policies.

“No data from GSR was used by Cambridge Analytica as part of the services it provided to the Donald Trump 2016 presidential campaign,” it said.

Kogan, meanwhile, said he felt scapegoated by Facebook and Cambridge Analytica. “Honestly we thought we were acting perfectly appropriately. We thought we were doing something that was really normal,” he told the BBC last Wednesday.

On March 19, the UK’s Channel 4 News broadcast undercover footage showing Cambridge Analytica executives – including CEO Alexander Nix – boasting they could entrap politicians with bribes and other ploys.

Nix was suspended by his firm a day later, though in a statement the company also said the Channel 4 News report had been edited to “grossly misrepresent the nature of those conversations”.

Meanwhile, questions have been raised about Cambridge Analytica’s involvement in other election campaigns.

The Observer published allegations the company had worked with people thought to be Israeli computer hackers in an unsuccessful attempt to sway the vote in Nigeria’s 2015 presidential election. Others raised questions about the role of Cambridge Analytica in shaping the outcome of Kenya’s disputed elections, in which Uhuru Kenyatta emerged victorious.

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On Tuesday, Wylie testified before UK parliamentarians about how pro-Brexit campaigners linked to Cambridge Analytica allegedly “cheated” in the run-up to the referendum on leaving the EU.

His nearly four-hour testimony followed allegations by a separate whistle-blower, Shahmir Sanni, that pro-Brexit campaign groups potentially broke laws on referendum spending limits when it paid data firm AggregateIQ, which is linked to Cambridge Analytica, for digital marketing services.

AggregateIQ reportedly received 40 percent of Vote Leave’s entire budget. The organisation’s campaign director Dominic Cummings said after the referendum victory, “We couldn’t have done it without [AggregateIQ].” According to the Observer, that quote was taken down from the company’s website on Thursday after being contacted by journalists.

Why are people angry at Facebook?

In the days following the original reporting, the Twitter hashtag #DeleteFacebook began trending. Congress asked Facebook CEO Mark Zuckerberg to testify on what had happened. The social media giant’s share value took a dramatic dive, down $60bn at one point down.

Facebook rejected the claim that a data breach had happened. “People knowingly provided their information, no systems were infiltrated, and no passwords or sensitive pieces of information were stolen or hacked,” it said in a statement.

When the data harvesting was taking place, Facebook’s policy allowed for the collection of friends’ data by app creators and academics, though selling this data to third parties or using it for advertising was not prohibited.

In 2015, Facebook found out Kogan’s company allegedly violated its policies when it passed the material on to Cambridge Analytica. In the March 16 statement, the social media company said it had been assured then that Cambridge Analytica, Kogan, and Wylie had deleted the data.

Still, the Observer reported: “At the time [Facebook] failed to alert users and took only limited steps to recover and secure the private information of more than 50 million individuals”.

Kogan’s app was removed from Facebook in 2015, but the social media behemoth didn’t suspend Cambridge Analytica and SCL Group – the firm that serviced contracts won by Cambridge Analytica and was called “effectively a shell” – until March 16, four days after the Observer contacted Facebook for comment.

Zuckerberg has since apologised for what he called a “breach of trust” in full-page newspaper ads in several UK and US newspapers.

On Tuesday, it was reported Zuckerberg will testify before US Congress on the Cambridge Analytica leak, though he declined to answer questions from UK lawmakers.

Irrespective of Facebook’s culpability, the scandal has tapped into ongoing concerns about how careful the platform is with users’ data – and how the company has been used to influence political outcomes.

Facebook has been under scrutiny over whether it did enough to stop Russian propaganda aimed at influencing the US election from spreading misinformation on users’ news feeds.

Reports of discontent among Facebook’s ranks have emerged. In November 2016, BuzzFeed reported that Facebook employees had formed “an unofficial task force” to create a list of recommendations to the company’s senior management about how to address the platform’s fake news problem.

Back then, Zuckerberg said the idea that fake news had affected the outcome of the US election was “a pretty crazy idea”.

This latest controversy does little to assuage fears that Facebook is unable or unwilling to prevent third parties from using it to further interests that go against those of its users.