Investigation: Keeping Things Secure | Walkthrough The Bureau Guide

Barnes can be found in Engineering.

This Investigation can be completed after mission Signal from Beyond. In the Engineering you have to find a pilot, Barnes. He tells you that some Russian tried to sabotage secret weapon of XCOM. Your task is to interrogate the spy, who is sitting in the room on right.

The good choice is "Work with me".

Conversation with Russian isn't difficult. First made choice doesn't matter and in the second one you have to choose the middle one, offering the spy a cooperation (screen above). Don't give up during conversation (left option) because it fails the mission.

As a reward for interrogation, you get additional distract mission: The Russian.