WASHINGTON — Mark Zuckerberg on Wednesday clashed with a second panel of congressional lawmakers who attacked the Facebook chief executive on a litany of issues, from user privacy to Russian propaganda and illegal opioid sales.

The five-hour hearing before the House Energy and Commerce Committee proved more tense than a marathon session in the Senate a day earlier. Democrats and Republicans alike repeatedly cut off Zuckerberg, who appeared less composed than he did at the Tuesday hearing. In all, Zuckerberg attended nearly ten hours of hearings.

Lawmakers once again threatened regulation if Facebook failed to improve its business practices. At one point in the hearing, though, Zuckerberg acknowledged that his own information was compromised as a result of the privacy controversy now looming over his company.

Opening the session, the House panel's chairman, Rep. Greg Walden, R-Ore., called Facebook an "American success story." But he added: "While Facebook has certainly grown, I worry it has not matured. I think it is time to ask whether Facebook may have moved too fast and broken too many things."

Driving lawmakers' scrutiny is a controversy around Cambridge Analytica, a political consultancy tapped by President Trump's 2016 campaign that improperly accessed the names, "likes" and other personal information of millions of Facebook users. For the first time, Zuckerberg said that his data was swept up by an app that fed data on 87 million users to Cambridge Analytica.

In the wake of its review of the firm's activities, Facebook also has acknowledged that malicious actors scraped information from the public profiles of practically its entire base, more than 2 billion users. Such scraping heightens the odds that Facebook could be subject to major fines from the Federal Trade Commission, which is investigating the matter, and it drew sharp rebukes from lawmakers who felt Facebook should have spotted it sooner.

"Facebook knew about this in 2013 and 2015, but you didn't turn the feature off until Wednesday of last week," Rep. Ben Ray Luján, D-N.M. said at one point during the hearing. "This is essentially a tool for these malicious actors to steal a person's identity and put the finishing touches on it."

Zuckerberg started the House hearing by repeating the same apology he gave to the Senate a day earlier. "It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he told House lawmakers.

Throughout the hearing, Zuckerberg's demeanor vacillated between calm and frustrated as lawmakers challenged the 33-year-old billionaire on a host of issues.

Rep. G.K. Butterfield, D-N.C., demanded that Zuckerberg improve the company's hiring practices, pointing out that Facebook had no people of color in its highest executive ranks. Rep. Joe Barton, R-Texas, meanwhile, pressed Zuckerberg on claims of bias against conservatives in the way his company handles content uploaded by its users.

Rep. David McKinley, R-W.Va., accused Zuckerberg and Facebook for "hurting people" by failing to combat users who try to sell opioids on the site. "I think there are a number of areas of content we need to do a better job of policing on our service," Zuckerberg replied.

In one of the toughest exchanges on Wednesday, Rep. Anna Eshoo, D-Calif. - who represents a slice of Silicon Valley - repeatedly needled Zuckerberg for failing to explain its data collection practices to users in "clear and pedestrian language."

Her Democratic colleague, Luján, raised reports that Facebook collects data on people who aren't even users - called "shadow profiles" by some. Zuckerberg, however, said he was "not specifically familiar with that." Nevertheless, Luján criticized Zuckerberg for a feature that allows web users who aren't signed up to learn more about the data collected by the social giant only if they become users.

"You're directing people who don't even have a Facebook page to sign up for a page to reach their data," Luján said.

Rep. Marsha Blackburn, R-Tenn., later remarked that Facebook looks "a whole lot like the 'Truman Show,'" where users' information is "made available to people they don't know, and then that data is crunched and used and they are fully unaware of this."

The Tennessee lawmaker cited laws that govern health data, financial transactions and other industries, before citing her bill that would require tech companies to obtain user permission before they can collect and sell user data. Facebook has long lobbied against the so-called Browser Act.

Repeatedly, though, lawmakers said the Facebook leader must provide greater clarity as to exactly how Cambridge Analytica obtained data on 87 million users in the first place. They warned a suit-clad Zuckerberg that tough regulation and scrutiny might follow if Facebook failed once again to improve its business practices.

"If all we do is have a hearing and nothing happens, then that's not accomplishing anything," said Rep. Frank Pallone Jr. of New Jersey, the top Democrat on the House Energy and Commerce Committee.

A day earlier, Senate lawmakers expressed the same fears.

"Unless there are specific rules and regulations enforced by an outside agency, I have no assurance that these kinds of vague commitments are going to produce action," Sen. Richard Blumenthal, D-Conn., said Tuesday during the Senate hearing.

"Mr. Zuckerberg, you've said you're sorry. I appreciate the apologies," added Sen. Catherine Cortez Masto, D-Nev., during later questioning. "But please stop apologizing and make the change."