Islamic State extremists have released a video purportedly showing the beheading of a second American journalist, Steven Sotloff.
The video includes a warning addressed to President Barack Obama that as long as US airstrikes against the militant group continue, "our knife will continue to strike the necks of your people".
The footage was posted two weeks after the release of a video showing the killing of James Foley and just days after Sotloff's mother pleaded for his life.
Barak Barfi, a spokesman for the family, said that the Sotloffs 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, a 31-year-old Miami-area native who freelanced for Time and Foreign Policy magazines, vanished in Syria in August 2013.
He was not seen again until he appeared in a video released last month that showed Foley's beheading. Dressed in an orange jumpsuit against an arid landscape, Sotloff was threatened in that video with death unless the US stopped airstrikes on the Islamic State (IS).
In the video distributed yesterday, titled "A Second Message to America," Sotloff appears in a similar jumpsuit before he is apparently beheaded by a fighter with the IS, the extremist group that has conquered large areas of territory across Syria and Iraq and declared itself a caliphate.
In the video, the organisation threatens to kill another hostage, who is said to be British.
In Washington, State Department spokeswoman Jen Psaki said US intelligence analysts will work as quickly as possible to determine if the video is authentic.
"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," she said. "Our hearts go out to the Sotloff family."
The fighter who apparently beheads Sotloff in the video calls it retribution for Obama's continued airstrikes against the group.
"I'm back, Obama, and I'm back because of your arrogant foreign policy toward the Islamic State … despite our serious warnings," the fighter says.
"So just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."
The killer specifically mentions the recent US airstrikes around the Mosul Dam and the beleaguered Iraqi town of Amirli, making it unlikely that Sotloff was killed at the same time as Foley, as some analysts had speculated.
Over the weekend, Iraqi government forces with help from US airstrikes broke the two-month IS siege of Amirli, a town where some 15,000 Shiite Turkmens had been stranded.
In a statement from US Central Command, military officials said an airstrike conducted on Monday against IS militants near the Mosul Dam damaged or destroyed 16 armed vehicles.
The SITE Intelligence Group, a US terrorism watchdog, first reported the video's existence.
In a sign of disorganisation – or perhaps dissension – in the extremist group's ranks, a faction of the IS apparently posted the video early, before it was supposed to be released. In a later Twitter message, those responsible apologised and asked fellow jihadis not to "reproach" them.
The IS has terrorised rivals and civilians alike with widely publicised brutality as it seeks to expand a proto-state it has carved out on both sides of the border.
In its rise to prominence over the past year, it has frequently published graphic photos and gruesome videos of bombings, beheadings and mass killings.
Last week, Sotloff's mother, Shirley Sotloff, pleaded with his captors for mercy, saying in a video that her son was "an innocent journalist" and "an honourable man" who "has always tried to help the weak".
Sotloff grew up in the Miami area, graduated from Kimball Union Academy, a school in New Hampshire, and then attended the University of Central Florida, which said he majored in journalism from 2002 to 2004 but apparently left without graduating.
Just how he made his way from Florida to Middle East hotspots is not clear. He published articles from Syria, Egypt and Libya in a variety of publications. Several focus on the plight of ordinary people in war-torn places.
In a statement, Foreign Policy magazine said it was saddened by news of his death and called him a "brave and talented journalist" whose reporting "showed a deep concern for the civilians caught in the middle of a brutal war".
Time Editor Nancy Gibbs said Sotloff "gave his life so readers would have access to information from some of the most dangerous places in the world".
House Foreign Affairs Chairman Ed Royce, a California Republican, said the new video demonstrates the Islamic State's "barbarity across the region – beheading and crucifying those who don't share their ideology".
He said the US and allies need to step up military action against the group, including through airstrikes.
At Sotloff's parents' home in Pinecrest, Florida., two police vehicles blocked the driveway, and officers advised journalists to stay away. Friends of the family could be seen coming and going.
"Everyone's been concerned. Everyone is grieving," neighbour Pepe Cazas said. "It's terrible. I've been praying for him."