The anonymous message board houses some of the most controversial content online. Here’s a look behind its rise in influence. (Video: Adriana Usero, Melissa Macaya, Deirdra O'Regan/The Washington Post)

The anonymous message board 8chan on Tuesday endured new attacks on multiple fronts, losing a key technical ally shortly before its owner was called before Congress to testify about the notorious site’s racist and extremist content.

The House Homeland Security Committee demanded in a letter Tuesday that 8chan owner Jim Watkins, an American Web entrepreneur living in the Philippines, provide answers on how the company had responded in the wake of three mass shootings this year promoted and celebrated on the site.

“Americans deserve to know what, if anything, you, as the owner and operator, are doing to address the proliferation of extremist content on 8chan,” said the letter, signed by chairman Rep. Bennie G. Thompson (D-Miss.) and Rep. Mike D. Rogers (Ala.), the committee’s top Republican. In a separate statement, the committee said lawmakers wanted to tackle “the very real and persistent threat from domestic terrorism.”

Read the Congressional letter demanding 8chan's owner to testify

The anything-goes message board remained offline Tuesday following major decisions by two tech companies to stop working with it.

It was further pummeled when Epik, a technology firm that had vowed to step in and provide assistance to bring the site online, issued a surprise reversal, citing “concern of inadequate enforcement and the elevated possibility of violent radicalization on the platform.”

Watkins did not immediately respond to requests for comment. Early Tuesday, he had posted a video to YouTube defending the site as “a peacefully assembled group of people talking.”

8chan vowed to fight on, saying its ‘heartbeat is strong.’ Then a tech firm knocked it offline.

The site has become infamous for its role in the fatal attacks in El Paso this weekend and earlier this year at mosques in Christchurch, New Zealand, and a San Diego-area synagogue. Before each shooting, authorities say, the suspects allegedly used the board to share hateful screeds explaining and promoting the attacks.

The board is almost entirely unmoderated, and its posters have routinely used the site to traffic in threats of violence and online hate. The site’s founder, Fredrick Brennan, told The Washington Post on Sunday that the site was a haven for domestic terrorists and should be shut down for good.

Three mass shootings this year began with a hateful screed on 8chan. Its founder calls it a terrorist refuge in plain sight.

On Tuesday, Brennan showed little sympathy for Watkins. “Jim totally deserves this; he wasn’t willing to even pretend to care or do the minimum after shootings to stop users inciting further violence," he said. "Given he hasn’t acted in good faith throughout this crisis, it’s ultimately a good thing for everyone, even 8chan’s own users, that 8chan is struggling to come back online.”

Epik, based outside Redmond, Wash., had said Monday that it would provide critical services to 8chan after its previous partner, the digital security firm Cloudflare, terminated its work with the site, calling it a “lawless ... cesspool of hate."

Epik chief Rob Monster had said Monday in a blog post that not working with 8chan was tantamount to an infringement of free speech. But on Tuesday, he issued a new post saying the company had drawn a “line on acceptable use.”

Shutting down a ‘cesspool of hate:’ Tech reporter Drew Harwell discusses 8chan on Post Reports