Authorities launched a search for two men from the Muslim-dominated Xinjiang region after three people drove an SUV into a crowd at Tiananmen Square and set it on fire on Monday.
They killed themselves and two tourists in the square, the heart of China's power structure and the focal point of the mass 1989 pro-democracy protests brutally crushed by the military.
Police have spread a dragnet across the capital, checking hotels and vehicles, seeking two people suspected to be ethnic Uighurs, a Muslim minority from Xinjiang in China's far west.
Two senior sources said the crash, which also injured 38 bystanders at perhaps the most closely guarded location in China, was suspected of being a suicide attack carried out by people from Xinjiang. It was initially believed to be an accident.
The sources did not specifically say the occupants were Uighurs, many of whom chafe at Chinese controls on their culture and religion.
There have been suicide bombings before in China, and in Beijing, mostly by people will personal grievances, but none has targeted the very heart of China's government.