Kim Hjelmgaard, USA TODAY

The British government said separately Wednesday that it made an unsuccessful attempt to rescue a British man David Cawthorne Haines being held by IS.

Speaking in Estonia on Wednesday where he is meeting with Baltic state leaders ahead of a NATO summit later this week in Wales, President Obama called the killing of Sotloff a 'horrific act of violence.'

'Whatever these murderers think they've achieved by murdering innocent Americans ... they have failed,' Obama said.

The president said Sotloff's killers would be brought to justice and that his death would only serve to unite Americans.

'Our reach is long and justice will be served,' Obama said.

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