DAMASCUS, Syria -- The 81-year-old antiquities scholar had dedicated his life to exploring and overseeing Syria's ancient ruins of Palmyra, one of the Middle East's most spectacular archaeological sites. He even named his daughter after Zenobia, the queen that ruled from the city 1,700 years ago.

That dedication may have cost him his life. On Wednesday, relatives and witnesses said Khaled al-Asaad was beheaded by Islamic State militants, his bloodied body hung on a pole in a main square.

Antiquities officials said they believed IS militants had interrogated al-Asaad, a long-time director of the site, trying to get him to divulge where authorities had hidden treasures secreted out of Palmyra before the extremists seized the ruins last spring.

The brutal killing stunned Syria's archaeological community and underscored fears the extremists will destroy or loot the 2,000-year-old Roman-era city on the edge of a modern town of the same name, as they have other major archaeological sites in Syria and Iraq.

"We have lost not just a scholar of archaeology but one of the pillars of archaeology in the 20th century," said Ahmad Ferzat Taraqji, a 56-year-old antiquities expert and friend of the victim.

The Sunni extremists, who have imposed a violent interpretation of Islamic law across the territory they control in Syria and Iraq, claim ancient relics promote idolatry and say they are destroying them as part of their purge of paganism -- though they are also believed to sell off looted antiquities, bringing in significant sums of cash.

Known as "Mr. Palmyra" among Syrian antiquities experts for his authoritative knowledge and decades administering the site, al-Asaad refused to leave even after IS militants captured the town and neighbouring ruins in May.

The Palmyra site was al-Asaad's life, said his nephew, an opposition activist who uses the name Khaled al-Homsi. Even when he could no longer go to the Roman ruins because of his advanced age, al-Asaad lived nearby, "and he could see the archaeological site from his house," al-Homsi told The Associated Press.

IS extremists detained the scholar three weeks ago, al-Homsi said, speaking on condition his real name not be used for fear of reprisals from IS and the Syrian government.

On Tuesday, al-Homsi watched as al-Asaad was brought in a van to a main square near a vegetable market packed with shoppers. Dressed in ordinary clothes and not the orange jumpsuits worn by other hostages before they were beheaded, al-Asaad stood as a militant read out five accusations against him, including that he was the "director of idols," represented Syria "at infidel conferences" and visited Shiite powerhouse Iran.

Then another militant pulled out a knife, at which point al-Homsi said he left the square, unable to watch. Al-Asaad's body was later hung from a pole on a main street, a paper outlining the "charges" against hung around his waist.

Maamoun Abdulkarim, the head of the Antiquities and Museums Department in Damascus, said al-Asaad was a pioneer in Syrian archaeology.

He said IS had tried to extract information from him about where some of the town's treasures had been hidden in order to save them from the militants.

Palmyra was a prominent ancient city-state under the rule of the Roman Empire. In the 3rd century, its queen, Zenobia, led a revolt against Rome that briefly succeeded in holding much of the region until it was crushed. The ancient remains, including temples and dramatic colonnades, are a UNESCO world heritage site.

The brutal killing and threat to the region's archaeological treasures brought international condemnation.

Speaking in Washington, State Department spokesman John Kirby said, "The United States condemns in the strongest possible terms this murder ... of a man who dedicated his life to preserving Syria's cultural treasures."

"Like so many of ISIL's victims, his life and extraordinary work stand in stark contrast to that of his barbaric killers. These attempts to erase Syria's rich history will ultimately fail," Kirby said, using an alternate acronym for IS.

Friends and colleagues of al-Asaad said Palmyra was his life.

"He was Mr. Palmyra, you couldn't do any work in Palmyra without going through him," said Amr al-Azm, an antiquities expert and professor at Shawnee State University in Ohio, adding that al-Asaad was indispensable to the management of the heritage sites.

"No one's been there for consistently so long and covered so many aspects of Palmyra's cultural heritage. I think it's irreplaceable," al-Azm said, speaking from Prague.

Al-Asaad was also a hard-core supporter of President Bashar Assad, and had been a member of Syria's ruling Baath party since 1954.

Al-Homsi, who is opposed to the Syrian president, said his uncle did not believe in the uprising against Assad's rule, which erupted in March 2011 and later turned into civil war. "He did not take it seriously."

Al-Asaad had been in charge of Palmyra's archaeological site for four decades until 2003, when he retired, according to Syria's state news agency SANA. He then worked as an expert with the Antiquities and Museums Department.

Al-Asaad, who held a diploma in history and education from the University of Damascus, wrote many books and scientific texts. Among his titles are "The Palmyra Sculptures," and "Zenobia, the Queen of Palmyra and the Orient."

He also discovered several ancient cemeteries, caves and the Byzantine cemetery in the garden of the Museum of Palmyra, the agency said.

"Al-Asaad was a treasure for Syria and the world," Khalil Hariri, al-Asaad's son-in-law who works at Palmyra's archaeological department, told the AP by phone from the central Syrian city of Homs. "Why did they kill him?"

Hariri said other antiquities officials who were detained by IS in the past two months had been questioned about where artifacts were hidden.

He and other colleagues said al-Asaad was central in the effort to move the contents of the Palmyra museum, including hundreds of artifacts and statues, to safety before the Islamic State group seized the town.

Hariri, who is married to al-Asaad's daughter, Zenobia, said his father-in-law is survived by six sons and five daughters.

Since falling to IS, Palmyra's ancient site has remained intact, though the militants destroyed a lion statue dating back to the 2nd century. The statue, discovered in 1975, had stood at the gates of the town museum.

In early July, IS released a video showing the killing of 20 captured government soldiers in Palmyra's amphitheatre. They were shot dead by young IS members armed with pistols. Hundreds of people were seen watching the killings.

"I begged him two months ago to leave the town and come to Damascus with his family, but he refused," said Taraqji, who is director of excavations at the antiquities department in Damascus.

"He believed in destiny," Taraqji said. "He told me, 'I was born in Palmyra and will stay in Palmyra and will not leave even if costs me my blood."'