ISIS 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, activists say. The killings took place on Monday at the Palmyra archaeological site, located a few miles away from the city, according to a local activist and the Syrian Observatory for Human Rights network. The identities of those slain remain unknown. (Earlier this month, ISIS blew up a 2,000-year-old "Arch of Triumph" in the city, which it seized from Syrian government forces in May.)