Islamic State extremists released the video showing Sotloff's beheading on Tuesday. It comes two weeks after the militants released a video showing the killing of James Foley, another American journalist.
In the Sotloff video, a masked militant warns President Barack Obama that continuing airstrikes against the group in Iraq will be met with the killing of more Western captives.
Sotloff is a 31-year-old Miami-area native who freelanced for Time and Foreign Policy magazines. He vanished in Syria in August 2013.
National Security Council spokeswoman Caitlin Hayden announced the U.S. intelligence community's assessment of the video in a statement Wednesday.