2nd US reporter beheaded

Islamic State extremists released a video Tuesday purportedly showing the beheading of a second American journalist, Steven Sotloff, and warning President Barack Obama that as long as U.S. airstrikes against the militant group continue, "our knife will continue to strike the necks of your people."

The footage — depicting what the U.S. said appeared to be a sickening act of brutality — was posted two weeks after the release of video showing the killing of James Foley and days after Sotloff's mother pleaded for his life.

Barak Barfi, a spokesman for the Sotloff family, said that the family had seen the video but that authorities have not established its authenticity.

"The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," Barfi said.

Sotloff, 31, who freelanced for Time and Foreign Policy magazines, vanished in Syria in August 2013 and was not seen again until he appeared in a video released online last month that showed Foley's beheading. Dressed in an orange jumpsuit against an arid Syrian landscape, Sotloff was threatened in that video with death unless the U.S. stopped airstrikes on the group in Iraq.

In the video distributed Tuesday and titled "A Second Message to America," Sotloff appears in a similar jumpsuit before he is apparently beheaded by a fighter with the Islamic State, the extremist group that has claimed wide swaths of territory across Syria and Iraq and declared itself a caliphate.

In the video, the organization threatens to kill another hostage, this one identified as a British citizen.

The SITE Intelligence Group, a U.S. terrorism watchdog, first reported about the video's existence. Unlike the footage of Foley's beheading, which was widely shared on Twitter accounts affiliated with the Islamic State group, the video purporting to show Sotloff's killing was not immediately posted online, though several jihadi websites told users to expect it Tuesday.

In Washington, State Department spokeswoman Jen Psaki said U.S. intelligence analysts will work as quickly as possible to determine if the video of the beheading is authentic.

"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," Psaki said. "Our hearts go out to the Sotloff family and we will provide more information as it becomes available."

Psaki said it's believed that "a few" Americans are still being held by the Islamic State but would not give any specifics.

The fighter who beheads Sotloff in the video called it retribution for Obama's continued airstrikes against the group in Iraq.

"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State ... despite our serious warnings," the fighter said. "So just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."

At the end of the video, he threatens to kill a third captive, a Briton, David Cawthorne Haines. It was not immediately clear who Haines was. Officials with the British Foreign Office declined to immediately comment.