WASHINGTON (AP) — Facebook and Twitter executives assured Congress on Wednesday that they are aggressively working to root out foreign attempts to sow discord in America, and they pledged to better protect their social networks against manipulation during the 2018 midterm elections and beyond.

Facebook's No. 2 executive, Sheryl Sandberg, and Twitter's CEO, Jack Dorsey, testified before the Senate intelligence committee in the morning, but there was an empty chair for Google parent company Alphabet, which refused to send its top executive.

In the afternoon, Dorsey went before a House panel alone to address Republican concerns that Twitter is censoring conservatives. Dorsey denied that is happening.

The hearings come at a critical time, just two months before the midterm elections and as President Donald Trump has charged that Twitter is biased against Republican views.

Senators had sharp words for Alphabet CEO Larry Page. Sen. Marco Rubio, R-Fla., suggested the company was a no-show because it was "arrogant."

Sandberg's appearance came several months after Facebook CEO Mark Zuckerberg testified at highly publicized Capitol Hill hearings.

Like Zuckerberg, she acknowledged Facebook's lag in recognizing Russian efforts to manipulate Facebook during and after the 2016 presidential election. Sandberg detailed Facebook's efforts to fight the problem with new technology and manpower.

"We are even more determined than our adversaries, and we will continue to fight back," she said.

Dorsey was candid with both committees about what his company needs to improve, while defending Twitter against allegations of bias.

Holding his phone throughout the hearings, Dorsey tweeted some of his opening statement to the Senate: "We aren't proud of how that free and open exchange has been weaponized and used to distract and divide people, and our nation. We found ourselves unprepared and ill-equipped for the immensity of the problems we've acknowledged."

He added: "Abuse, harassment, troll armies, propaganda through bots and human coordination, misinformation campaigns, and divisive filter bubbles — that's not a healthy public square. Worse, a relatively small number of bad-faith actors were able to game Twitter to have an outsized impact."

As the executives spoke, the Justice Department announced that it would look at whether their companies are hurting competition and "intentionally stifling the free exchange of ideas on their platforms."

Justice Department spokesman Devin O'Malley said Attorney General Jeff Sessions will meet with a number of state attorneys general later this month to discuss the department's concerns.

Sandberg, 49, has extensive Washington experience, typically acts as her company's public face and clearly felt comfortable answering to the senators. The bearded and tieless Dorsey, 41, is far less of a public figure and was quiet and respectful in his answers. Both contrasted with Zuckerberg's sometimes awkward defiance during his Washington appearance in April.

Thirteen Russians were indicted by special counsel Robert Mueller this year on charges of taking part in a plot to disrupt the 2016 election by creating fake social media accounts that pushed divisive issues.

Both Facebook and Twitter are using artificial intelligence and other increasingly sophisticated technology to combat manipulation. Facebook is going after "inauthenticity," or fake accounts. Twitter is focusing on analyzing behavior patterns to find suspicious activity because Twitter technically allows "fake" accounts.

The companies have made many policy changes and have caught and banned malicious accounts over the past year. Still, their business models — free services that rely on attracting as many users as possible for as long as possible and finding out as much about them as possible — remain the same, and that has posed challenges in rooting out those bent on mischief.

GOP Sen. Richard Burr of North Carolina, the Intelligence committee chairman, commended the companies for their efforts but said Congress is concerned that not enough has been done.

"Clearly, this problem is not going away," Burr said. "I'm not even sure it's trending in the right direction."

Dorsey said Twitter has continued to identify accounts that may be linked to the same Russian internet agency cited in Mueller's indictment. He said Twitter has suspended 3,843 accounts it believes are connected to that agency. Facebook has also taken down pages this year that it believes were tied to the agency.

At the House hearing, Energy and Commerce chairman Greg Walden, R-Ore., cited recent complaints that Twitter limited the visibility of prominent Republicans on its platform — a charge echoed by Trump himself.

"It takes years to build trust, but it only takes 280 characters to lose it," Walden said.

Dorsey has strongly denied that political ideology has played a part in any Twitter algorithms that determine what users see.

The assertion that conservatives are being censored has also been pushed by House Majority Leader Kevin McCarthy, R-Calif., and the No. 3 House Republican, Steve Scalise of Louisiana.

Committee Democrats charged that Republicans were manufacturing the issue ahead of the November elections.

"This hearing appears to be one more mechanism to raise money and generate outrage," said Rep. Frank Pallone of New Jersey, the committee's top Democrat. Other Democrats called it a "charade" and "a load of crap."

Twitter came under fire from some on the far right after suspending conspiracy theorist Alex Jones last month, as did Facebook. Jones made an angry appearance Wednesday outside both hearing rooms, telling reporters he was there to "face my accusers."

Jones railed against the government, media and social media companies for more than 40 minutes in the hallway as the Senate hearing began. Later, he heckled a reporter outside the House hearing room.