The execution of Sotloff, 31, came despite pleas from his mother aimed directly at ISISs top leader seeking mercy for her son, a freelance journalist who was captured in northern Syria a year ago.
Word of Sotloffs beheading came two weeks after James Foley, 45, another American journalist, was beheaded by ISIS, which warned that Sotloff would be the next to die.
The videotaped beheadings and threats by ISIS have helped rocket the groups ascendance into a top crisis for the Obama administration and its Western allies.
Josh Earnest, a White House spokesman, said he could not immediately confirm the authenticity of the video showing Sotloffs killing. It is something that will be analyzed very carefully by the US government and our intelligence officials to determine its authenticity, he told reporters in Washington.
In the video, Sotloff describes himself as paying the price for the Obama administrations decision to strike ISIS targets in Iraq. The same masked fighter who appeared in the video of Foleys beheading also appears beside Sotloff, asserting, Im back, Obama, and Im back because of your arrogant foreign policy towards the Islamic State.
The SITE group also said ISIS was threatening a third captive, a Briton it identified as David Cawthorne Haines, as the next beheading victim.
At least two other Americans are believed to be held by ISIS.
Sotloff was a Florida native who wrote for Time magazine and other publications, and who had reported on the convulsions of the Middle East for the past few years. His capture by ISIS was largely kept a secret for months at the request of his family until he was seen in the execution video of Foley two weeks ago.
It was unclear from the video of Sotloff where and when it was recorded. Some Western intelligence officials have said they believe Sotloff was executed the same day as Foley, and that the propagandists at the ISIS decided to space out the publicity of each one.
Last Thursday Sotloffs mother, Shirley, released an emotional video appeal to his captors, asking the leader of ISIS, Abu Bakr al-Baghdadi, the self-proclaimed caliph of the Muslim world, to grant amnesty to her son. I ask you to use your authority to spare his life, Sotloff implored.
- RICK GLADSTONE