ISIS Threatens 'Slaughter' after Kassig Execution

An act of "pure evil" is how President Barack Obama described Islamic State's latest brutal beheading of an American, 26-year-old aid worker Peter Kassig.

Unlike other beheading videos, the footage doesn't show Kassig's murder. Rather, his severed head was shown lying at the feet of his executioner.

Kassig's murderer, a man known as Jihadi John, taunted President Obama in the video.

"And here we are burying the first American crusader in Dabiq," the masked jihadist says.

Dabiq is the Syrian city where Islamic State jihadists believe they will have a final showdown with Western forces, an Armageddon of sorts that will usher in a worldwide Islamic caliphate.

"Today we are slaughtering the soldiers of Bashar," an unidentified ISIS jihadist says in the video. "And tomorrow we will be slaughtering your soldiers and with Allah's permission we will break this final and last crusade and the Islamic State will soon, like your puppet David Cameron said, begin to slaughter your people on your streets."

Kassig's executioner is believed to be the same man appearing in other ISIS videos. He's thought to be a Westerner because of his notable British accent.

"It does seem to be the same person. And, you know, obviously inside the government is working very hard to exactly identify him in anything we can learn about him," former National Counterterrorism Center director Matt Olsen said.

The Kassig beheading footage gave the FBI some important clues about Jihadi John's possible whereabouts because landmarks are clearly visible in the background.

Meanwhile, Kassig's execution may not be the last of a kidnapped American.

The Islamic State is now threatening to behead another U.S. aid worker - this time a young female captured in Syria. And if ISIS carries out her execution on video, that would be a first for the terrorist group which has never shown the execution of a Western woman.

She's the last American they currently hold. The woman's family has asked that the media not identify her because the attention may place her life in greater jeopardy.

Meanwhile, Secretary of Defense Chuck Hagel said the United States will accelerate training of Iraqi troops to combat ISIS. U.S. Special Forces are now in Anbar province leading the training effort.

It comes at a time when a senior Kurdish leader now estimates the number of ISIS troops at 200,000 -- possibly 10 times larger than earlier estimates.