The leaders of the House Homeland Security Committee subpoenaed the owner of online message board 8chan on Wednesday for testimony on the proliferation of harmful content on social media sites, a move that comes as the platform faces scrutiny for serving as a haven for extremists.

The subpoena issued by committee Chairman Bennie Thompson, a Democrat from Mississippi, and ranking member Mike Rogers, a Republican from Alabama, calls for Jim Watkins, owner of the online message board, to appear before the panel Sept. 5.

“In recent years, violent extremist content has proliferated on both large and small social media platforms,” Thompson and Rogers said in a statement. “At least three acts of deadly white supremacist extremist violence have been linked to 8chan in the last six months.”

The lawmakers said they want to know what Watkins is doing to counter the spread of hateful content on his platform.

Thompson and Rogers sent Watkins a letter last week requesting he testify before the Homeland Security Committee and explain what he is doing to address the proliferation of extremist posts on 8chan.

Watkins’ online message board has come under criticism in the wake of the Aug. 3 shooting at a Walmart in El Paso, Texas, which left 22 dead and two dozen wounded. Police believe the suspected shooter, 21-year-old Patrick Crusius, penned an anti-immigrant screed in which he declared the attack a “response to the Hispanic invasion of Texas.”

The manifesto was posted to 8chan before the shooting rampage began.

The massacre in El Paso hardly marked the first time postings on the online message board were traced back to the alleged perpetrators of violent acts.

Police believe the gunman who opened fire at a synagogue in Poway, California, in April published an anti-Semitic letter on 8chan before the shooting. The suspected gunman responsible for attacks at two mosques in Christchurch, New Zealand, was also found to have uploaded a 74-page manifesto to 8chan.

Watkins, however, defended the platform in a video last week and said his company has cooperated with law enforcement when a threat of violence is posted.