PHOENIX (AP) — Authorities searched Monday for whoever planted a pipe bomb in a truck, triggering a lockdown at the nation's largest nuclear power plant, after concluding the vehicle's owner was no longer a suspect.

Authorities shifted their attention away from 61-year-old Roger William Hurd, who returned to work at the plant Monday, to "find out who really made the bomb and put it in his truck," Maricopa County Sheriff Joe Arpaio said.

Investigators are interviewing residents at Hurd's apartment complex in Goodyear, about 24 miles west of Phoenix, the sheriff said. "We want to catch the perpetrators that caused this havoc," Arpaio said.