BEIRUT (AP) — Activists say the Islamic State group has killed three captives in Syria's ancient city of Palmyra by tying them to Roman-era columns at the site, then blowing the structures up with explosives.

A Palmyra activist who goes by the name Nasser al-Thaer said on Tuesday that the killings took place the day before at the Palmyra archaeological site, located a few kilometers (miles) away from the city. Al-Thaer and the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, say the identities of those slain remain unknown.

IS captured Palmyra from government forces in May. Since then, it has destroyed ancient temples and other relics.

The group is known for gruesome killings, including beheadings and immolation.