The House Committee on Homeland Security subpoenaed the owner of 8chan, James Watkins, to testify before the committee in September. In a press release, the committee said it wants Watkins to testify Sept. 5 on matters related to “extremist content” on social media.

“In recent years, violent extremist content has proliferated on both large and small social media platforms. At least three acts of deadly white supremacist extremist violence have been linked to 8chan in the last six months,” the committee said in a statement. “We have questions on what is being done to counter this trend so we can be sure it is being properly addressed.”

The New Zealand mosque shootings and the Poway synagogue shooting were all announced in advance on the online message board. 8chan’s founder Fredrick Brennan told The New York Times he’d like to see it shut down. Watkins, a U.S. military veteran, has run the site since 2015 after Brennan gave it up. Watkins has said he would keep the website dark until he spoke with the homeland security committee.