Washington Post: Surveillance footage, cellphone data led to takedown of Austin bomber, officials say

The FBI agents and police investigators tasked with identifying who was responsible for planting a series of bombs here seemed at times to be chasing ghosts.

They ran down theories of drug-dealer retaliation gone awry and struggled to understand the significance of family connections between the victims. All the while, the bomber escalated his attacks — first by using a tripwire and then by sending explosives through FedEx.