Wellinghoff, who was in office during the incident, said he reached his conclusion after consulting with Defense Department experts about the attack that involved snipping AT&T fiber-optic lines to knock out phone and 911 service, and firing shots into a PG&E substation, causing outages.
"This is the most sophisticated and extensive attack that's ever occurred on the grid to my knowledge," Wellinghoff told The Associated Press. Similar statements were published on Tuesday by The Wall Street Journal.
FBI spokesman Peter Lee said Wednesday that at this point "we don't believe it's an act of terror, international or domestic."
The FBI is the lead agency in the investigation, and Lee said the agency has specific definitions of terrorism that involve motive, intent and political ideology. He said the investigation is ongoing and urged people not to jump to conclusions.
Wellinghoff said he was speaking out on the issue now because he's concerned the grid isn't being adequately protected.
He said he didn't know what the motivation was for the sabotage, "but I don't know what the definition of terrorism is other than when an extremely well-trained group attacks a major piece of infrastructure in an expertly planned attack.
His concerns underscored earlier statements from high-ranking officials about the incident.
In October, former CIA director Jim Woolsey said in a Commonwealth Club appearance that video from the incident showed a group of three or four men, in a "disciplined military fashion," had conducted the attacks. He provided details about how they systematically fired their weapons, and said they "quickly and professionally disposed of everything they had."
"This wasn't hooliganism," he said. "This was a systematic attempt to take down the electric grid."
In December, during an oversight hearing, U.S. Rep. Henry Waxman, D-Calif., described "an unprecedented and sophisticated attack on an electric grid substation with military-style weapons. Communications were disrupted. The attack inflicted substantial damage."
He said he would withhold details of the incident to avoid harming the investigation but added he had been in touch with the FBI about it.
Last April, a day after the Boston Marathon bombings, millions of people in Santa Clara County were asked to conserve energy after power lines were damaged.
At a news conference later that day, Sheriff Laurie Smith said someone had lifted heavy manhole covers at about 1:30 a.m. in two places on Monterey Highway south of San Jose, climbed under the road, and cut AT&T fiber optic cables, temporarily knocking out 911 service and phone service.
About 15 minutes later, someone fired a high-powered rifle into a nearby PG&E substation, damaging at least five transformers and causing an oil leak, she said.
"We believe that the perpetrator or perpetrators were familiar with the systems," she said then. "They knew where to go for the fiber-optic. They knew where to cut. They also were able to take out some very, very critical parts of the PG&E substation."
She said the attacks could be described as "sabotage."
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