The C.I.A. and an Afghan intelligence official said that the shooter had been part of an Afghan “partner force,” known as a Zero Unit, trained and supported by the agency in the southern province of Kandahar. The Afghan refugee accused of shooting two National Guard members in Washington, D.C., fought in the late days of the U.S. war there as part of a “Zero Unit,” a paramilitary force that worked with the C.I.A., according to a person briefed on the investigation and an Afghan intelligence officer familiar with the matter. The units were known for their brutality and labeled “death...