Twitter is suspending the accounts of anyone who tweets "graphic imagery" of journalist James Foley's execution, CEO Dick Costolo announced Tuesday.

Islamic militants released a video online Tuesday allegedly showing the beheading of Foley, a freelance journalist who was kidnapped by gunmen in 2012 in Libya. At the end of the video, Steven Joel Sotloff, the name of another American journalist who was kidnapped in Syria in July 2013, appears on screen, and the man who beheaded Foley says "the life of another American citizen, Obama, depends on your decision.

The video, titled "A Message to America," was removed from YouTube, but screenshots from the footage continued to spread on social media.

"We have been and are actively suspending accounts as we discover them related to this graphic imagery," Costolo tweeted Tuesday. He linked to a New York Times article about Foley without an image.

The move came after Kelly Foley, Foley's sister, tweeted "Please honor James Foley and respect my family's privacy. Don't watch the video. Don't share it. That's not how life should be." Her account has since been made private.

Friends and colleagues of Foley posted memorials to the journalist, and asked people to circulate photos from Foley's life to remember him, not photos from his death.

Meanwhile, Wednesday's New York Post and Daily News both printed images from the video on their cover, showing the masked man with a knife over Foley with the headline "Savages."