ISIS supporters took to social media to share a photograph purporting to show the body of 30-year-old Tomislav Salopek, who has been held by militants in Sinai province since July 22 after being kidnapped in the capital Cairo.
The gruesome image appears to show Mr Salopek's severed head placed on his back - a trademark of ISIS militants who have brutally beheaded at least seven Western hostages over the past year.
Buried in the sand next to his bloodied corpse is the chilling black and white flag used by ISIS.
The 30-year-old oil and gas sector surveyor is the first foreign captive to be killed in this way in Egypt since the extremists set up a branch in the country.