BEIRUT — The bodies of at least 67 Syrian civilians, many summarily killed by the Islamic State group, have been discovered in a central town in Syria that government forces retook from the extremists over the weekend, the Syrian government and activists said Monday. A senior Syrian official described the attack as a “shocking massacre,” saying the search for those killed in the town of Qaryatayn is still underway, and the body count was likely to climb. Some were shot in the street as the militant extremists retreated from the town because they were suspected of working with the government, according to activists.