A mass grave of people believed to have been executed in the Soviet Union in the 1930s was uncovered Wednesday near Ulan-Ude, Russia, investigators said. The remains of at least 16 people were found during housing construction on the outskirts of the Siberian city, in the republic of Buryatia, when a worker spotted a human skull in the digging bucket of an excavator. Investigators found more remains, with wounds characteristic of execution-style gunshots to the back of the head, the news agency RIA Novosti reported Wednesday. Historical archives indicate more than 20,000 Buryatia residents were killed during Stalinist repression in the 1930s, with at least 6,000 executed by firing squads in 1937-38, RIA Novosti said.