ISIS terrorists operating in Syria murdered 33 young men on Wednesday in what human rights activists have dubbed a mass execution.

The 33 victims ranged in age from 18 to 25, though their identities have yet to be released.

According to the Syrian Observatory for Human Rights, a group which monitors war crimes by both the Assad regime and terrorist groups like ISIS working to overthrow the Syrian government, the mass killing is the “largest execution operation” by ISIS this year.

The Observatory also noted that the murdered men were killed with “sharp tools”, and that “marks of slaughter” were found around their necks.

The 33 bodies were discovered in a mass grave that was “filled with blood”, the Observatory reported.

On Tuesday, a chemical attack on a suburb of the city of Idlib in northwestern Syria left roughly 100 dead, mostly civilians, among them women and children.