Mark ZuckerbergCEO of Facebook Mark Zuckerberg, Washington, USA - 09 Apr 2018CEO of Facebook Mark Zuckerberg (C) is surrounded by Capitol Police officers as he walks down a hallway to the office of Democratic Senator from California Dianne Feinstein, on Capitol Hill in Washington, DC, USA, 09 April 2018. Zuckerberg is meeting with lawmakers before testifying in two Congressional hearings this week regarding Facebook allowing third-party applications to collect the data of its users without their permission, and for the company's response to Russian interference in the 2016 US presidential election.

In Mark Zuckerberg’s written Congressional testimony, released Monday, the Facebook chief executive officer apologized for his company’s failure to block improper use of people’s data and vowed to take correctional steps — the latest of which has begun rolling out.

“It’s clear now that we didn’t do enough to prevent these tools from being used for harm. That goes for fake news, foreign interference in elections and hate speech, as well as developers and data privacy,” Zuckerberg said in a prepared testimony made public by the U.S. House Committee on Energy and Commerce.

While last year’s headlines focused on Facebook’s role in disseminating fake news, the latest controversy stems from London-based Cambridge Analytica and its unauthorized use of personal Facebook data in the 2016 presidential election. Last week, the social giant admitted that the records of as many as 87 million users may have been affected. The tech ceo is slated to appear before Congressional committees on Tuesday and Wednesday.

The written remarks echo Zuckerberg’s call with reporters last week, when he took the blame for his company’s inadequacies. He wrote, “We didn’t take a broad enough view of our responsibility, and that was a big mistake. It was my mistake, and I’m sorry. I started Facebook, I run it and I’m responsible for what happens here.”

During the company’s third-quarter earnings call with investors in November, before the Cambridge Analytica issue became public knowledge, the Facebook founder spoke of plans to invest deeply in security, going as far as saying that these efforts would affect profitability, rattling Wall Street.

At the time, Facebook employed 10,000 people to work on safety and security. Five months later, on last week’s press call, Zuckerberg said the number is up to 15,000 and pledged the workforce dedicated to the area would hit 20,000 by year’s end.

He also told reporters that he’d give users a way to find out if they were among the millions whose data was pilfered by Cambridge Analytica.

The launch of that effort started on Monday and will come in stages so not all users will see the new tool immediately. Those who do will get one of two possible notices at the top of their News Feeds — either a new section explaining that their information may have been shared with Cambridge Analytica, with a link to “see how you’re affected,” or a generic message linked to privacy controls. The latter allows people to check which apps are connected to their accounts, see the info they’re accessing and revoke permissions, if they choose.

Cambridge Analytica has disputed Facebook’s estimate, but even Zuckerberg seems appalled at the scale of the controversies plaguing his company. On Friday, the tech company supported legislative proposals aimed at social media sites, including mandatory disclosures of exactly who is buying political ads and new verifications for parties pushing ads focused on gun control, racism and other hot-button issues.

In his prepared testimony to Congress, Zuckerberg said his company was “too slow to spot and respond to Russian interference, and we’re working hard to get better.”

The U.S. Federal Trade Commission is also investigating Facebook’s data practices.