He looked after the ancient ruins of Palmyra for 40 years. Now ISIS has killed him.

Syrian archaeologist Khaled al-Asaad was brutally killed Tuesday by ISIS. The militant group beheaded Asaad, the foremost expert on Palmyra history, and then hanged his body from one of the Greco-Roman columns in the ancient city.

The octogenarian archaeologist had been held for about a month by ISIS and was killed after refusing to give up information on the whereabouts of Palmyra artifacts, according to Syria's director of antiquities, Maamoun Abdul Karim.

"Al-Asaad was a treasure for Syria and the world," Khalil Hariri, Asaad's son-in-law, who works at the Palmyra's archaeological department told The Associated Press. "Why did they kill him?"

Amr al-Azm, an associate professor in Middle Eastern history and anthropology at Shawnee State University in Ohio who worked for a period with Asaad, added that Asaad dedicated his life to working at ancient archaeological sites.

“Because he’d spent so many years working on this site, he was so familiar with the archaeology of the area and the city, he was a huge repository of knowledge, all acquired first hand just by being there, and working it," Azm says. "And really this vast repository of information has now been lost to us. And it’s not the kind of information you can acquire by reading a book or attending a lecture, it’s all very practical knowledge and information. And it’s all gone now.”

Asaad helped evacuate many of Palmyra’s movable artifacts as ISIS approached. But he decided to stay when ISIS arrived.

“The trucks were literally rolling out of the city from one end as ISIS were breaking into the city from the other," Azm says. "He could have jumped on those trucks and left with the artifacts. But he chose to stay instead. And I suppose paid the ultimate price for that.”

Palmyra, considered an archaeological jewel of the Middle East and a UNESCO World Heritage site, was seized by ISIS in May. The ancient city was an important trading hub along the Silk Road.

Asaad had worked over the past few decades with US and European archaeological missions on excavations and research in Palmyra and was widely recognized as an expert on the ancient ruins there.

There are fears that ISIS will destroy Palmyra. The group has very publicly demolished several ancient sites already in Iraq.