House Democrats have issued a subpoena for the owner of the anonymous online forum 8chan to testify before Congress on how to mitigate online extremism on his platform.

The House Homeland Security Committee issued a subpoena for Jim Watkins, the owner of the 8chan forum. The House Democrats have ordered him to appear before the committee on September 5.

“At least three acts of deadly white supremacist extremist violence have been linked to 8chan in the last six months,” the committee leaders said in a statement Wednesday. “We have questions on what is being done to counter this trend so we can be sure it is being properly addressed. Receiving testimony from Mr. Watkins is critical to our oversight on this matter.”

Last week, House Homeland Security Committee Chairman Bennie Thompson (D-MS) and Mike Rogers (R-AL) sent a letter to Jim Watkins, asking him to testify before the committee.

The lawmakers hope to discuss Watkins’ efforts to “investigate and mitigate the proliferation of extremist content,” which includes “white supremacist content” on 8chan.

The two lawmakers wrote the letter to the 8chan owner in the wake of the August 3 attack in El Paso, Texas, which resulted in the deaths of 22 Americans. The killer reportedly wrote his manifesto describing anti-immigrant and “white supremacist beliefs” on 8chan before the shooting, which Watkins denied in a video posted to YouTube.

In a separate statement, Chairman Thompson said, “White supremacist terrorism … is on the rise and is now our top domestic terrorism threat.”

Watkins has lived in the Philippines but said that he was planning to visit the United States in a video posted last week.

“It’s a shame I’m being blamed for this,” he said in the video.

In the wake of the El Paso shooting, Internet infrastructure provider Cloudflare suspended its servicing of 8Chan and called the online forum a “cesspool of hate.” Cloudflare CEO Matthew Prince also banned the white nationalist The Daily Stormer, claiming in 2017 that it would not set a “precedent.”

The lawmakers noted last week that Fredrick Brennan, the founder of 8chan, called for the forum to be shut down.