A Chinese official said on Tuesday that police had arrested 18 members of a separatist terrorist group who had been trained outside China. The authorities meanwhile stepped up Olympics security following Monday’s deadly attack in a western Chinese city.

Two men used knives and explosives on Monday to kill 16 police officers and injure 16 more in the predominantly Uighur Muslim city of Kashgar, one of the worst incidents of political violence in China for a decade.