The Islamic State group killed three of its 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, activists said Tuesday.

It was the latest gruesome method of killing by the militant group, which has become known for horrific beheadings and even immolation of its prisoners.

Earlier this week, the extremist group posted images on social media purported to show its members driving a tank over a captured government soldier, allegedly to revenge what it said was his driving over IS militants. The IS militant group is known to have tanks, mostly captured in battle from Syrian troops or in the territory it holds in neighboring Iraq.

IS has also destroyed many of the ancient Palmyra’s Roman-era relics, including the magnificent Temple of Bel and the iconic Arch of Triumph. IS captured Palmyra from government forces in May, and considers such relics as promoting idolatry. But scientists say the group resorts to promoting and documenting such attacks also for their shock value, to spread awe and fear. The group has relied on looting and selling such antiquities on the black market for revenue.

A Palmyra activist who goes by the name Nasser al-Thaer said that the killings of the three took place the day before in the afternoon 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, said the three were civilians but that their identities remain unknown.