Left-wing online streamer and political commentator Hasan Piker condemned a “culture of violence” in the United States following the murder of conservative activist Charlie Kirk.

“In less than two weeks, I was supposed to debate Charlie Kirk,” Piker wrote in a New York Times op-ed published on Saturday.

Piker said he was live-streaming when he learned about Kirk’s gruesome killing on Wednesday and was “shocked” by what he saw.

“It was the horror of seeing someone whom I know — not a friend or an ally, but a human being I know personally and have debated before — fall victim to what clearly seems to be a rising tide of political violence,” he wrote.

As of Saturday, the motives of Kirk’s killer were unclear. Shell casings from the weapon used to shoot Kirk were covered in messages that appeared to reference internet meme culture and video games.