A sophisticated sniper attack in April that riddled PG&E’s Metcalf power substation in South San Jose with bullets may have been an act of domestic terrorism, two experts say, underlining concern that the nation’s electricity grid is vulnerable to sabotage.

While the FBI says there is no evidence that terrorists were involved, Jon Wellinghoff, former chairman of the Federal Energy Regulatory Commission, said the attack was “very well planned and well executed by very highly trained individuals,” a conclusion shared by a former top PG&E official. Wellinghoff added that “a coordinated attack could put this country in a world of hurt for a long time.”

Based on his review of the evidence and a tour of the Metcalf plant with some military experts, he said the assault was “the most significant incident of domestic terrorism involving the grid that has occurred” in North America.

But the FBI, which is the primary agency looking into the incident — doesn’t share his conviction.

“We do not believe it is related to domestic or international terrorists,” said FBI spokesman Peter Lee, noting that the case is still under investigation and no one has been arrested. He added that there is no evidence linking it to several other attacks on the power grid in Arkansas, where a man undergoing psychiatric evaluation was charged with the crimes last year.

PG&E spokesman Brian Swanson declined to discuss the matter in detail or comment on whether officials at the San Francisco-based utility believe terrorists were involved, adding, “we won’t speculate about possible motives until the investigation is complete.”

Wellinghoff, who now works for a law firm in San Francisco after resigning from the Federal Energy Regulatory Commission in November, said he believes the commission also is investigating the Metcalf incident. But the agency’s spokeswoman, Mary O’Driscoll, said she couldn’t comment on that.

As a result of the attack, PG&E shut down much of the substation for nearly a month while repairs were made. During that time, power was routed to customers through other PG&E equipment.

A former PGE official shares Wellinghoff’s concerns. Mark Johnson, a retired PG&E vice president, told an industry gathering in November that the incident might have been a rehearsal for a bigger attack later, according to a report in The Wall Street Journal on Wednesday.

“This wasn’t an incident where Billy-Bob and Joe decided, after a few brewskies, to come in and shoot up a substation,” he was quoted as saying. “This was an event that was well thought out, well planned and they targeted certain components.”

The attack at the electricity transmission substation — located next to the Metcalf power plant — happened shortly before 2 a.m. on April 16 last year, when one or more individuals took up positions along Coyote Ranch Road and “began shooting rounds at the equipment, according to a California Public Utilities Commission report. Reportedly firing bullets like those used by AK-47s — an assault weapon favored by terrorists — they blasted 17 transformers and 6 circuit breakers, and caused $15.4 million in damage,

Although no one was injured and no one lost power, the circumstances of the attack suggest it was carefully planned and carried out with precision.

The attackers severed six AT&T fiber optic telecommunication lines in an underground vault, which was covered by a metal lid that was so heavy it would have taken at least two people to lift it, according to sources quoted by The Wall Street Journal. Besides more than 100 fingerprint-free shell casings littering the area, Wellinghoff said his military experts spotted small rock piles by the plant which the attackers might have placed there earlier to mark prime firing positions.

So far, no group has publicly claimed responsibility for the attack or made demands as a result of it. That seems inconsistent with the FBI’s official definition of terrorism, which partly hinges on the perpetrators attempting to “influence the policy of government by intimidation or coercion.” Even so, the attack has drawn considerable attention.

During a congressional committee hearing in December, Rep. Henry Waxman, D-Los Angeles, said the incident makes it “clear that the electric grid is not adequately protected.”

It also has led to numerous meetings among federal law enforcement, national security and energy officials to emphasize the need to protect the power grid from sabotage, according to Scott Aaronson, senior director of national security policy for the Edison Electric Institute, an industry group.

He stressed that utilities already do a lot to safeguard their equipment, which includes thousands of electricity transmission substations like the one at Metcalf. Nonetheless, he added, “these incidents are always opportunities for the industry to get better at security.”

Contact Steve Johnson at sjohnson@mercurynews.com or 408-920-5043. Follow him at Twitter.com/steveatmercnews.