Xin Chun was a former nuclear technician turned terrorist. He wasn't paid much so he quit and starting selling nuclear secrets to developing nations. He built dirty bombs for an al-Qa'ida cell in Syria before returning to North America to work freelance.
He was part of a group that attempted to blow up the Pickering power station in Canada. When the Seventh Doctor revealed that one member of their group, Laura Collinson, was actually a CIA agent, Xin ran off, only to be stopped by the military, who had stormed the building. (PROSE: The Nuclear Option)