BEIRUT, Lebanon — For decades, he was the bespectacled caretaker of some of Syria’s greatest archaeological treasures. He explored the sprawling ruins in his hometown, named a daughter Zenobia after its ancient queen, and became so intertwined with its development that one historian called him “Mr. Palmyra.”

Now, months after his home fell to the jihadists of the Islamic State, Khalid al-Asaad, the retired chief of antiquities for Palmyra, has fallen, too.

After detaining him for weeks, the jihadists dragged him on Tuesday to a public square where a masked swordsman cut off his head in front of a crowd, Mr. Asaad’s relatives said.

His blood-soaked body was then suspended with red twine by its wrists from a traffic light, his head resting on the ground between his feet, his glasses still on, according to a photo distributed on social media by Islamic State supporters.