How many criteria are there for what is and what is not a terrorist attack?

US politicians use one criterion when they associate incidents, such as the fatal bombing in Boston, with terrorist groups. But when it comes to the killing of innocent residents by terrorists in China's Xinjiang and Tibet autonomous regions, it seems a different criterion applies.

When terrorists killed 24 innocent people, including two police officers, in Shanshan, Xijiang, on Wednesday, rather than showing sympathy to the victims and indignation toward the perpetrators, the US State Department spokesman urged China to "provide all Chinese citizens - including Uygurs - the due process protections to which they are entitled."