SHANGHAI—Chinese authorities on Tuesday said an August explosion at a factory that supplied auto parts for General Motors Co. and other car makers killed at least 146 people, nearly double the initial death toll.

The Chinese government also said late Tuesday that it would prosecute three senior executives of Kunshan Zhongrong Metal Production Co., the company that owned the factory in the eastern industrial city of Kunshan where the explosion occurred, as well as 15 Kunshan government officials. The statement added that Guan...