A former US Federal Energy Regulatory Commission chairman says attackers who shot at an electricity substation in April 2013 were well trained.
A former regulator says a shooting that nearly knocked out Silicon Valley's power supply was a terrorist attack, possibly in preparation for a bigger strike.
The Federal Bureau of Investigation (FBI) said on Friday it could not rule anything out, but the 2103 attack on an electricity substation near San Jose was more likely a criminal incident, rather than terrorism.
Shooters using semiautomatic weapons caused extensive damage to 17 transformers in the attack in the early hours of April 16.
At the time, site owner Pacific Gas & Electric Co played down the incident as vandalism.
Nevertheless, power company bosses scrambled to avoid a potentially disastrous blackout in California's Silicon Valley, which is home to some of the world's biggest technology companies.
Jon Wellinghoff, who chaired the Federal Energy Regulatory Commission at the time, said this week the attackers were clearly well trained and tried to cause major damage.
"This event shows there are people out there capable of such an attack," Wellinghoff said on Thursday, cited by The Wall Street Journal.
"It would not be that hard to bring down the entire region west of the Rockies if you, in fact, had a co-ordinated attack like this against a number of substations."
An FBI spokesman played down the possibility that the attack on the Metcalf substation was terrorism-related.
"I cannot tell you 100 per cent that this is not terrorism," Peter D. Lee of the FBI's San Francisco field office said.
"However, there are certain other things that we take into consideration when we investigate whether it is criminal or national-security related... And we are treating it as a criminal matter."
Asked if the FBI could rule out terrorism, he replied: "We can never do that... We don't rule anything out just in case. But right now we don't think it's terrorism."
A spokesman for Pacific Gas & Electric Co did not immediately respond for comment.