Facebook’s CEO spent five hours on Tuesday alternately answering and deflecting questions about user privacy and how the social network handles your data.

Hear, hear: Zuckerberg’s testimony before a joint hearing of the Senate Commerce, Science, and Transportation and Senate Judiciary committees was sparked by the revelation weeks ago that Facebook is at the center massive data scandal.

The background: It was revealed in March that millions of its users’ information was shared with Cambridge Analytica—a firm that provided data to President Trump’s election campaign in 2016. Facebook now estimates that many as 87 million users were affected. 

This is just the latest in a string of data privacy-related issues the world’s largest social network is dealing with.

What happened in Washington: In a room packed with politicians, reporters, photographers, and some sign-wielding protesters, Zuckerberg first read from prepared testimony, noting several of the things Facebook is doing to address data privacy problems and prevent abuses like Cambridge Analytica’s from happening again, such as further limiting developers’ access to users’ information.

He then spent hours answering senators’ questions, including one from Kamala Harris of California about why Facebook didn’t let users know about issues related to Cambridge Analytica back in 2015 (which is when it originally learned that the company had bought data from a researcher and demanded it be deleted—though it’s now clear that’s not what ended up happening). Zuckerberg wouldn’t say whether Facebook made a clear decision not to inform users but said that, looking back, the company screwed up in how it handled the situation. (See also “Facebook may stop the data leaks, but it’s too late: Cambridge Analytica’s models live on.”)

In response to many of the questions, rather than offering immediate answers Zuckerberg said he’d have his “team” get back to senators with details.

A few interesting points: in a response to a question by Vermont senator Patrick Leahy, Zuckerberg said Facebook is cooperating with special counsel Robert Mueller’s investigation into Russian interference in the 2016 US presidential election.

He also said that while Facebook is beefing up its use of AI to spot and remove offensive content, it will be five to 10 years before the company will have tools to do this automatically.

Many senators seemed unsure about how Facebook works—for instance, Zuckerberg had to say several times that Facebook doesn’t share or sell user data to advertisers (rather, it allows advertisers to tell it what kinds of users they want to target on Facebook). This lack of understanding points to a broader problem the company has long struggled to address: many of its users are simply not as tech-savvy (or Facebook-savvy) as those behind the social network.

What’s next: This was the first of two hearings on Zuckerberg’s schedule. On Wednesday, he will speak before the House Energy and Commerce Committee; you can watch at the committee's website.