President Barack Obama says Americans will not be intimidated by the "horrific" violence of Islamic State militants after the beheading of a second U.S. journalist.

The president is vowing that "justice will be served" after the U.S. verified the authenticity of a video of freelance reporter Steven Sotloff's killing.

The video comes just two weeks after another video showing the beheading of another U.S. reporter, James Foley.

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.

In the Sotloff video, a masked militant warns Obama that continuing airstrikes against the group in Iraq will be met with the killing of more Western captives.

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