The Federal Bureau of Investigation (FBI) said yesterday it could not rule anything out, but that the attack on an electricity substation near San Jose was more likely a criminal matter, rather than terrorism-related.
Shooters using semi-automatic weapons caused extensive damage to 17 transformers in the attack in the early hours of April 16. The site's owner, Pacific Gas & Electric Co, downplayed the incident as vandalism at the time.
Nevertheless, power company bosses scrambled to avoid a potentially disastrous blackout in California's Silicon Valley, home to some of the world's biggest technology companies.
"This event shows there are people out there capable of such an attack," Wellinghoff said Thursday, cited by The Wall Street Journal.
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"It would not be that hard to bring down the entire region west of the Rockies if you, in fact, had a coordinated 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 percent that this is not terrorism," Peter D. Lee of the FBI's San Francisco field office told AFP.
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 Company did not immediately respond for comment.