Contrary to initial accounts, the bomb did not explode. Nogales police Lt. Carlos Jimenez described it as a crude incendiary device that could fit in a person's hand. It was placed under the valve of the diesel tank and ignited, charring the steel tank.
"They were able to gain access to the facility illegally," Jimenez said. "They had some working knowledge of what that tank is or how it works."
The attackers failed to understand that diesel has a high flash point and is difficult to ignite.
Police identified no suspects or witnesses. They said there were no signs of vandalism common with domestic extremist groups such as the Earth Liberation Front, or ELF. The FBI has designated the ELF as a domestic terrorism group, which the bureau blames for arson attacks on homes, power facilities and other symbols of urbanization.
Nogales officials called the FBI, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives and the state Department of Public Safety for help. Those agencies would not comment, citing the ongoing investigation.
Police said they believe the saboteurs got into the substation sometime between 4 p.m. Tuesday, when maintenance workers locked it and left, and 8 a.m. Wednesday, when workers returned to monitor the plant.
Had there been a catastrophic explosion, as many as 30,000 customers could have lost power, Jimenez said.
The Valencia Generating Station is a small "peaking" facility, used only during the hottest hours of summer or the coldest hours of winter, when electricity demand spikes. The adjacent substation, however, is important for balancing the regional power supply.
The plant is owned by UniSource Energy Services, a subsidiary of UNS Energy, which is also the parent company of Tucson Electric Power.
Company spokesman Joe Salkowski said the four turbines at the 63-megawatt plant were not running at the time of the incident.Electricity supplies were not affected by the event or subsequent evacuation, he said.
The plant has fewer than five workers on site, with two more in nearby offices, he said.