Images posted online seem to show a beheaded Tomislav Salopek, a Croatian national, officials from various countries and the SITE Intelligence Group
As has been the case before with ISIS beheadings, there was no immediate or definitive proof such as DNA evidence that Salopek had been killed. CNN could not independently confirm the authenticity of the image.
Still, the image and a video released days ago threatening just a horrific act had many fearing the worst.
"We cannot confirm with 100% certainty that it is true," Croatian Prime Minister Zoran Milanovic said. "... However, what we have seen ... doesn't look good. It looks horrible."
Video demanded release of female Muslim prisoners
Salopek was kidnapped July 22 in the Cairo area, according to his employer and the Croatian government.
The Croatian foreign ministry said last month
that armed men stopped and kidnapped a Croatian national with the initials T.S. while he was driving to work for a French-owned company. The ministry identified the captive in subsequent news releases as Salopek.
In a video distributed online August 5, a group claiming to be ISIS' branch in the Sinai Peninsula threatened to kill Salopek
if Egypt didn't release female Muslim prisoners in 48 hours.
Kneeling in a jumpsuit in front of a masked, knife-holding man in camouflage, the hostage identified himself as Salopek and read from a paper, saying he was captured July 22 and worked for a French geoscience firm.
Ardiseis Egypt, part of the French firm CGG, said the hostage in the video appears to be Salopek, who it says was a subcontracted worker.
'This is an attempt to shatter our way of living'
ISIS has used this script before: taking a civilian from another country hostage, threatening to kill them, then carrying out a beheading when their demands aren't met. The killings have been part of the Islamist terrorist group's propaganda campaign, often publicized in videos.
U.S. journalist James Foley
was one of the first to meet such a fate, in 2014. Many others have followed since then, including American Steven Sotloff
and British aid workers David Haines
and Alan Henning
While such killings are not unprecedented, that doesn't mean they are any less shocking. That was the case again Wednesday, with the White House, British Foreign Secretary Philip Hammond and others offering condolences and condemnation.
The latest apparent atrocity was met with shock and horror.
"It is with horror that I am learning about the announcement from the Daesh terrorist group of the execution of a Croatian hostage," French Foreign Minister Laurent Fabius said in a statement, referring to ISIS by another name. "If confirmed, this awful assassination would demonstrate once again the cowardly nature and barbarity of this terrorist organization."
The reaction was strongest, and most personal, in Croatia.
Speaking from the Adriatic city of Split, Croatian President Kolinda Grabar Kitarovic -- who was NATO assistant secretary general for public diplomacy before taking office last winter -- said that Croatian authorities that as long as there is "one little crumb of a chance that Tomislav is alive, we are continuing ... the search."
Still, Milanovic conceded that "we fear that unfortunately what's happened to other countries' citizens has befallen this Croatian citizen."
The Croatian leader said the beheading won't spur his country to step up its engagement going after ISIS "because others need to handle that." Nor, he said, should it change how citizens approach the world; doing so would give ISIS exactly what it wants.
"We have to proceed with normal life," Milanovic said, "because this is an attempt to shatter our way of living."