(AP) The White House says it has determined that a video showing the brutal killing of freelance journalist Steven Sotloff is authentic.



Islamic State extremists on Tuesday released the video showing Sotloff's beheading. 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.



The president meanwhile said that Americans will not be intimidated by the "horrific" violence of Islamic State militants. He's vowing that "justice will be served." 



Obama said at a news conference while traveling Wednesday in Estonia that their deaths only unite Americans. He says Americans will not forget and the country's reach is long.

Sotloff is a 31-year-old Miami-area native who freelanced for Time and Foreign Policy magazines. He vanished in Syria in August 2013.