By Benjamin Kang Lim and Ben Blanchard BEIJING (Reuters) - Chinese authorities investigating what could be Beijing's first major suicide attack searched on Tuesday for two men from Muslim-dominated Xinjiang after three people suspected to be from the restive region drove an SUV into a crowd at Tiananmen Square and set it on fire. They killed themselves and two tourists on Monday in the square, the heart of China's power structure and the focal point of the mass 1989 pro-democracy demonstrations brutally crushed the military. ...