Former officials said the C.I.A. diligently assessed those who partnered with its forces, like the man accused of shooting two National Guard members in Washington.

In Webster Springs, everyone knows one another. When National Guard Specialist Sarah Beckstrom was killed, that connection was even further strengthened.

Prosecutors say members of the Somali diaspora, a group with growing political power, were largely responsible. President Trump has drawn national attention to the scandal amid his crackdown on immigration.