ISIS terrorist (illustrative)
ISIS terrorist (illustrative)Screenshot

American aid worker Peter Kassig may have struggled to the end with his captors, experts have speculated.

Kassig, who reportedly converted to Islam while in captivity and took on the name Abdul Rahman, is the fifth western hostage to be executed by the Islamic State (ISIS) terrorist group in recent months.

A video released by ISIS Sunday morning claimed to show Kassig's decapitated head, and featured a terrorist who appeared to be the same masked executioner who featured in previous videos - nicknamed "Jihadi John" due to his clear British accent.

But the similarity to previous videos ends there. The footage does not take place against the same desert landscape as all four previous video of executions, and is less slickly-made - leading some analysts to speculate that ISIS's capabilities have been somewhat hampered by the US-led military campaign. 

Most tellingly, however, is the fact that Kassig does not make any statement to the camera prior to his execution, in stark contrast to previous videos in which orange jumpsuit-clad hostages are forced to issue statements against their governments before being beheaded in identical, gruesome rituals.

Instead, Kassig's bloodied head is simply shown lying on the floor between "Jihadi John"'s feet.

"This is Peter Edward Kassig, a US citizen of your country. Peter, who repeatedly fought against the Muslims of Iraq while serving as a soldier under the American army, doesn’t have much to say. His previous cellmates have already spoken on his behalf," the masked Islamist says.

Later Sunday, US President Barack Obama confirmed the tape's authenticity.

Peter Kassig was "taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said. "Today we offer our prayers and condolences to the parents and family of Abdul-Rahman Kassig, also known to us as Peter."

Speaking to Britain's Telegraph, security expert Will Geddes speculated that Kassig, a US army veteran, may have realized that he was going to be killed whether or not he cooperated with his captors - and chose to resist.

"This young guy was a former ranger, he had operated as a combatant in Iraq. He may have got wind through one of his jailers or other prisoner that they were pulling out British and European hostages. 

"He could have said there’s no way I am going to propagate any of your message through a televised address," said Geddes.

Terrorism expert Jean-Charles Brisard agrees.

"Obviously there was something that happened during the filming," Brisard told the New York Times, adding that it was possible Kassig had struggled with his captives.

He also noted that Kassig's killing was not in fact the central aspect of the video, and only came towards the end. Again, unlike previous clips of executed western hostages - which were just a few minutes long and focused entire on the hostages and their executions - this one lasted for more than 15 minutes, most of which had nothing to do with Kassig.

The video began with a brief history of the ISIS terrorist group, and continues with a graphic mass-execution of men said to be captured Syrian army soldiers.

Led in a line by ISIS terrorists, each of the men is forced to pick his own execution knife from a long box before all are beheaded in a highly-choreographed scene.

Only the last two minutes feature "Jihadi John" and his victim Peter Kassig.

"This is why I am pretty sure that the real focus is not on Peter Kassig," said Brisard.

The video also differs from previous clips in that no subsequent hostage is brought to the camera and threatened with execution if the US and UK do not cease their anti-ISIS operation, perhaps indicating that ISIS has realized its tactic has not succeeded in stopping the western-led offensive against it.

But Professor Peter Neumann of King’s College Centre for Study of Radicalisation and Political Violence suggested that detail could in fact indicate that ISIS is running out of western hostages to kill.

One western hostage who does remain is John Cantlie, a British photojournalist who has appeared in several ISIS propaganda films directed as "news" reports. But Cantlie could still have other uses for ISIS, Neumann speculated.

"I don’t think they have decided yet what to do with Cantlie, he must be still quite useful for them."