The House Homeland Security Committee subpoenaed the owner of the website 8chan on Wednesday to testify before lawmakers next month, heightening the scrutiny on an anonymous message board that is notorious for online extremism and connected to three mass shootings this year.

The bipartisan subpoena of Jim Watkins, an American Web entrepreneur living in the Philippines, marks the most significant action yet taken against the site, which has been offline for more than a week after Internet infrastructure firms accused the board of spreading online hate and stopped providing services to it.

“In recent years, violent extremist content has proliferated on both large and small social media platforms,” the committee leaders Rep. Bennie Thompson (D-Miss.), its chairman, and Rep. Mike D. Rogers (R-Ala.), said in a statement Wednesday.

“At least three acts of deadly white supremacist extremist violence have been linked to 8chan in the last six months. 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.”

Adam Comis, a Thompson spokesman, said Wednesday that his office had been in contact with Watkins and was “hoping for full compliance.” Watkins’s testimony — possibly in a closed-door deposition, Comis said — is scheduled for the morning of Sept. 5.

The committee sent a letter last week requesting Watkins come testify before lawmakers. But the congressional subpoena, signed by Thompson, gives that demand greater weight and raises the possibility that the U.S. Marshals could help pursue Watkins if he does not comply.

Watkins did not respond to requests for comment. In a video posted to YouTube last week, he defended the site as a “peacefully assembled group of people talking … now looking for a home.”

In a separate video posted Sunday, Watkins said he had taken 8chan offline voluntarily and would bring it back online after he talks to “Homeland Security.” The video is tagged as being filmed at the Sands Regency Casino Hotel in Reno, Nev., near where 8chan parent company N.T. Technology is based.

It’s unclear where Watkins is staying. A representative of that hotel Wednesday said no guests have checked in under his name. In a tweet last week, 8chan shared a Watkins email in which he said he was on his “way back to America as we speak.”

In the live-streamed video, Watkins took questions from viewers and said he wants lawmakers “to understand we have nothing to do with this crazy violence that’s been happening."

“It is a First Amendment issue, the fact that the government is blaming me. That’s why I have it off, for that reason,” he added, before a short pause. “I need to probably talk to a lawyer, huh?”