Sean Holstege

The Republic | azcentral.com

A makeshift bomb exploded at a Nogales, Ariz., power plant Wednesday morning, damaging a large fuel tank and prompting an investigation by the FBI and federal bomb experts.

Local officials were alerted at 9:30 a.m. to "suspicious activity" at UniSource Energy Services' Valencia Plant. The explosion damaged a diesel storage tank, but there were no reports of injuries and authorities said they knew of no suspects or witnesses.

Officials closed off the power plant and an adjacent car dealership on North Grand Avenue. The FBI, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the Arizona Department of Public Safety were called.

Wednesday evening, agents were still processing the scene. Arizona Corporation Commission spokeswoman Rebecca Wilder said there were no power disruptions and the plant received only minor damage.

"The reason for the high-scale response is the plant is an electrical substation and critical to the area," Nogales police Lt. Carlos Jimenez said, explaining that as many as 30,000 customers depend on the plant for power.

"The whole city of Nogales could have been compromised," he added.

Authorities described the explosive as "a suspicious device" but would not elaborate. The fuel did not ignite, Jimenez said.

Plant operator UES is a subsidiary of Tucson-based UNS Energy Corp., the parent of Tucson Electric Power. UES provides energy to 237,000 customers in Arizona, according to the company website.