Special Agent Diana Chen ducked under the yellow tape surrounding what remained of Substation 12. The smell of burnt metal and melted plastic filled her nose. Five days after the explosion, the site still smoldered in places. FBI evidence techs in white suits moved through the wreckage, marking and collecting anything that might help identify who had done this.

"What do we have so far?" her partner, Agent Trent Morgan, asked as he joined her, notebook in ...