Steven Joel Sotloff, the U.S. journalist last seen being threatened by the Islamic State of Iraq and Syria, appears to have been beheaded in a video released Tuesday by the Islamist extremist group.

The video was obtained by the SITE Intelligence Group, which monitors extremist activity online and on social media.

Sotloff vanished in Aleppo, Syria, in 2013. He was shown on a video released Aug. 18 that claimed to show the beheading of another U.S. journalist, James Foley, who also went missing in Syria in 2012, as retaliation for the U.S. bombing campaign against ISIS. The video said Sotloff would be next if President Obama did not reverse course.

U.S. officials on Aug. 20 confirmed the authenticity of the Foley video, and later revealed that an attempt to rescue the hostages had failed because they were not where intelligence had revealed they would be.

In a video released last week, Sotloff's mother, Shirley, pleaded with ISIS leader Abu Bakr al-Baghdadi to release her son.