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The ISIS terror group has published a video titled "A second message to America," showing the beheading of American journalist Steven Sotloff.

The video also threatens the life of British captive David Haines.

Related: ISIS Militants Execute British Aid Worker David Haines; Cameron Expresses Outrage

Sotloff speaks to the camera before he is killed, saying he is "paying the price" for U.S. intervention. Considering he was a captive, it's possible that his words were scripted for him.

The masked ISIS figure in the video speaks to U.S. President Barack Obama, telling him, "Just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."

The Islamic State in Iraq and Syria has thrived and mutated during the civil war in Syria and in the security vacuum that followed the departure of the last American forces from Iraq. The aim of ISIS is to create an Islamic state across Sunni areas of Iraq and in Syria. The group has taken up large swaths of land in Iraq and has said it wants to go into Baghdad.

The intelligence community in the United States is working to confirm the authenticity of the video appearing to show Sotloff.

"If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends," said Bernadette Meehan, National Security Council deputy spokeswoman.

British Prime Minister David Cameron, responding to a reporter's question about the video, said, "I've just seen the news. It's an absolutely disgusting and despicable act and I will be making a statement later."

Sotloff's family knows about the video and "is grieving privately," said their spokesman Barak Barfi.

Beheading intended to energize followers

Last week, Sotloff's mother, Shirley Sotloff, released a video pleading with ISIS leader Abu Bakr al-Baghdadi not to kill her son.

"Steven is a journalist who traveled to the Middle East to cover the suffering of Muslims at the hands of tyrants. Steven is a loyal and generous son, brother and grandson," she said. "He is an honorable man and has always tried to help the weak."

Steven Sotloff appeared last month in an ISIS video showing the decapitation of another American journalist, James Foley. The militant in the video warned that Sotloff's fate depended on what Obama did next in Iraq.

Sotloff disappeared while reporting from Syria in August 2013, but his family kept the news secret, fearing harm to him if they went public. Out of public view, the family and government agencies had been trying to gain his release for the past year.

Speaking on CNN moments after word of video showing Sotloff's apparent murder, CNN terrorism analyst Paul Cruickshank said that the method of killing -- beheading -- has a specific purpose for ISIS.

Complete Coverage: James Foley

A video like the one showing Sotloff's killing "really energizes" supporters of ISIS, and beheading is employed for "maximum propaganda" to "terrify" ISIS' enemies, Cruickshank said.

Who was Sotloff?

Sotloff, 31, grew up in South Florida with his mother, father and younger sister. He majored in journalism at the University of Central Florida. His personal Facebook page lists musicians including the Dave Matthews Band, Phish, Miles Davis and movies including "Lawrence of Arabia" and "The Big Lebowski" as favorites. On his Twitter page, he playfully identifies himself as a "stand-up philosopher from Miami."

In 2004, he left UCF and moved back to the Miami area.

He graduated from another college, began taking Arabic classes and subsequently picked up freelance writing work for a number of publications, including Time, Foreign Policy, World Affairs and The Christian Science Monitor. His travels took him to Yemen, Saudi Arabia, Qatar and Turkey -- among other countries -- and eventually Syria.

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Response from the White House

CNN's Christiane Amanpour said that Sotloff's killing will step up pressure on Obama to devise a strategy to combat ISIS. Several other experts agreed with Amanpour.

On Friday, Obama said it was too soon to discuss what steps the U.S. would take against the militant group inside Syria. On how to deal with the group in Syria -- where it was born and has a haven, mostly in the city of Raqqa -- the President said: "We don't have a strategy yet."

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Obama said Friday that he had asked America's top defense officials to prepare "a range of options."

On Tuesday, White House spokesman John Earnest spoke to reporters just after word came about Sotloff's killing.

"This is something that the administration has obviously been watching very carefully since this threat against Mr. Sotloff's life was originally made a few weeks ago," Earnest said. "Our thoughts and prayers first and foremost are with Mr. Sotloff and Mr. Sotloff's family and those who worked with him."

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