Congressional investigators on Thursday issued subpoenas to Twitter, Reddit and the parent companies of Facebook and YouTube and accused them of failing to provide complete information on how their platforms spread falsehoods that fomented the Jan. 6, 2021, attack on the U.S. Capitol.

The move, a sharp escalation in a long-brewing standoff between investigators and the companies, came after months of seeking data that yielded “inadequate responses,” the chairman of the House committee investigating the Capitol siege, Rep. Bennie G. Thompson (D-Miss.), said.

“We cannot allow our work to be delayed any longer,” Thompson said in a statement. “Two key questions for the Select Committee are how the spread of misinformation and violent extremism contributed to the violent attack on our democracy, and what steps — if any — social media companies took to prevent their platforms from being breeding grounds to radicalizing people to violence.”

Thompson said that, despite requests for information sent in August, “we still do not have the documents and information necessary to answer those basic questions.”

Spokespeople for YouTube and Facebook disputed the characterization of their companies as not cooperating.

“We’ve been actively cooperating with the Select Committee since they started their investigation, responding substantively to their requests for documents, and are committed to working with Congress through this process,” said YouTube spokeswoman Ivy Choi.

Facebook spokesman Andy Stone offered similar assurance: “Meta has produced documents to the committee on a schedule committee staff requested — and we will continue to do so.”

Twitter spokeswoman Elizabeth Busby and Reddit spokeswoman Sandra Chu acknowledged that their companies had received the subpoena; Busby declined further comment, Chu said Reddit will “continue to work with the committee on their requests.”

Trump and his supporters used social media sites to spread falsehoods about the validity of Joe Biden’s victory over Trump in the November 2020 presidential election, with many posts calling for extraordinary action — including violence, in some cases — to prevent Biden from ascending to the White House. On Jan. 6, Congress was meeting to certify the election’s results, a procedural formality, when hundreds of Trump supporters bashed their way into the building as part of clashes that left five people dead.

No social media company has provided a full account of its role in spreading those falsehoods, and on Aug. 26, the committee requested information from 15 companies seeking data on what their internal studies had shown about the role their platforms had in the Jan. 6 attack and its origins. The August letters requested evidence be submitted by Sept. 9.

In a letter to Mark Zuckerberg, the CEO of Meta, Facebook’s parent company, Thompson said the committee had issued follow-up requests to Facebook on Sept. 28 and Oct. 29 but that Facebook has “declined to commit to a deadline for producing or even identifying" the materials sought.

Thompson made a similar complaint to the CEO of Twitter, Parag Agrawal, writing that “After over four months of good-faith negotiations ... it has become clear that Twitter is unwilling to commit to voluntarily and expeditiously complying with the Select Committee’s requests.”

In his letter to Reddit CEO Steve Huffman, Thompson acknowledged that Reddit had shut down its most contentious Trump-supporting subreddit on June 29, 2020. But he said that “Reddit still has not committed to a thorough review of its records for documents relevant to the Select Committee’s investigation and has refused to produce internal documents to support its conclusory public statements that Reddit played no role in January 6th.”

A committee aide said the committee is still negotiating with other tech companies that were sent information requests in August. “The Select Committee is in various stages of engagement with the other companies and efforts are ongoing to obtain information that will advance the committee’s investigation,” the aide said.

The House select committee investigating the attempted insurrection at the Capitol on Jan. 6 faces an uphill battle with former Trump administration officials. (Blair Guild/The Washington Post)

Demands for answers about social media’s role in the Jan. 6 violence have come mainly from Democrats, and the companies have sought to play down their role while quietly resisting demands for evidence that might provide a fuller picture. An investigation by ProPublica and The Washington Post recently found that more than 650,000 posts in Facebook groups attacked the integrity of the election — a pace averaging more than 10,000 a day — in the weeks leading up to the Capitol attack. Many posts called for arrests, executions and other violence against those seeking to keep Trump from maintaining the presidency in the face of his electoral loss.

In their efforts to regulate the tech industry, lawmakers have at times threatened to use subpoena power to force companies to hand over key documents. But such tactics are widely viewed as last resorts, and lawmakers typically prefer to pressure companies to voluntarily turn over data through letters.

Thompson had said as recently as October that the committee was “negotiating” with Facebook and the other companies, telling CBS’s Face the Nation, “It’s clear that the Jan. 6 organization, per se, used them as an organizing tool. To the extent that we can identify what will happen, that’s the committee’s charge from the House of Representatives. We’ll do it. But at this point, Facebook is working with us to provide the necessary information we requested.”

The committee’s patience, however, ran out this week.

In his letter to Zuckerberg, Thompson said the committee had specifically asked Facebook for information about reports that the company had disbanded a Civic Integrity Team that was focused on election misinformation after the election. The team’s dissolution has been reported by multiple sources, including The Post.

Thompson’s letter to Sundar Pichai, the CEO of Alphabet, YouTube’s parent company, said YouTube had broadcast “significant communications by its users that were relevant to the planning and execution of January 6th attack on the United States Capitol.” The letter noted that former Trump adviser Stephen K. Bannon had live-streamed his podcast on YouTube in the days before and after Jan. 6, 2021, and that images of the Capitol insurrection were carried live on YouTube.

“The Select Committee believes Alphabet has significant undisclosed information that is critical to its investigation, concerning how Alphabet developed, implemented, and reviewed its content moderation, algorithmic promotion, demonetization, and other policies that may have affected the January 6, 2021 events,” the letter said.

In his letter to Twitter, Thompson noted that "Twitter has not produced important documents relating to warnings it received regarding the use of the platform to plan or incite violence on January 6th.”

In his letter to Reddit, Thompson noted that while it had shut down TheDonald subreddit, Reddit continued to host other forums related to Trump, and shut down the biggest remaining one on Jan. 8, after the attack.

Gerrit De Vynck and Cat Zakrzewski contributed to this report.