SAN FRANCISCO » A mysterious and sophisticated sniper attack last year on a Silicon Valley power substation has underscored concerns about the vulnerability of the country’s electrical grid and prompted debate over whether it was an act of terrorism.

The chain of events is not in dispute: Shortly before 1:30 a.m. on April 16, 2013, one or more people methodically cut communication cables near a Pacific Gas & Electric substation in San Jose, sprayed more than 100 rifle bullets and knocked out 17 of the station’s 23 transformers before fleeing and avoiding capture. A grainy black-and-white surveillance video released by the Santa Clara County Sheriff’s office in a search for leads shows shots being fired for about a minute at the substation.

Though the utility was able to prevent a power failure by diverting electricity from other areas, the damage took 27 days to repair, said Brian Swanson, a spokesman for Pacific Gas & Electric.

The FBI has been investigating the attack, but says it has no evidence of terrorism nor any suspects.

"The FBI at this time does not believe it is related to terrorism based on the initial assessment of the investigation," Peter Lee, an agency spokesman in San Francisco, said, adding that he was unable to disclose further details. The agency also considers the attack an isolated one, Lee said.

"There was an incident in Arkansas, but at this time we believe it is separate," he said, referring to several episodes of sabotage last fall against the power grid in central Arkansas, for which a 37-year-old man was charged in November.

But Jon B. Wellinghoff, who was chairman of the Federal Energy Regulatory Commission at the time of the San Jose attack, said in an interview Wednesday, "I believe this was, in essence, terrorism," adding that the attack was carried out by "a group of individuals who were intent upon disrupting parts of the grid."

Debate over the attack was prompted by a Wall Street Journal article published Wednesday that took an in-depth look at the episode, which was a topic of discussion at a congressional hearing in December and was examined by Foreign Policy magazine the same month.

A law enforcement official briefed on the investigation said the situation so far was ambiguous.

"When you don’t know who did it and you don’t know what their motives were, it is very hard to say whether it was terrorism or not," the official said. "Some people said it looks like they had military training, some people say that you can learn this from a video game. We just don’t know."

With few witnesses and little other evidence, the FBI’s investigation has made little progress. In the coming weeks, the bureau may have to change its tactics and reach out to the public for help in identifying suspects.

Wellinghoff said that he had brought some experts from the Naval Support Facility Dahlgren, who train Navy SEAL units, to San Jose and that they had told him "they believed this was a very professional, very well organized, well thought out and well-executed action that took place."

Based on that assessment, "this could have been a dry run" for an even bigger attack, said Wellinghoff, a former Nevada consumer advocate who is now a lawyer in San Francisco.

The attack has renewed anxiety over the potential vulnerability of the power grid to physical attack, adding to worries about cybersecurity and the ordinary adversaries of hurricanes, floods, wild animals and falling trees.

On Wednesday, utility officials attempted to tamp down concern.

"It’s harder to knock out the lights than people think because of redundancy and resilience," said Gerry W. Cauley, president of the North American Electric Reliability Corp., a nonprofit group that sets standards for the nation’s utilities.

Substations like the one attacked in San Jose are clusters of transformers that change the voltage of electricity, increasing it to higher levels for transmission and reducing it to lower levels for distribution. At high voltage, line losses are smaller.

The three power grids in North America — one covering Texas, and one each covering the eastern and western portions of the United States and Canada — have thousands of substations.

Wellinghoff said that the ones most urgently needing protection were the ones connecting transmission lines of various high voltages, and that this was a "limited number," but he would not say what it was.

The substation hit in San Jose, he said, ranked No. 45 in California, meaning it was not critical. But the attackers disabled 17 of 23 transformers at the site, he said.

Most of the substations are owned by publicly traded utilities; a few are owned by government agencies. Wellinghoff said, "To my knowledge, there hasn’t been a comprehensive plan developed" to defend them.

The location of substations is public, but it is a closely guarded secret what combination of them would have to be knocked out to cause extensive harm. It could be as few as a handful in each of the three grids, the eastern continent, from Halifax to New Orleans, the western continent, from New Mexico to Vancouver, and Texas.

In response to the April attack, the nation’s electric utilities began a 2 1/2-year program to identify what substations or combinations of substations were most critical to the operations of the continent’s three power grids, how to minimize damage to them once an attack was detected, how to bring in law enforcement personnel before sending in the repair crews, and how to reconfigure the system after an attack to achieve maximum capacity.

Some of the emergency steps were exercised in a drill that the industry and government agencies held late last year to simulate cyberattack, physical attack and other grid threats.

Among the steps is changing where power is generated. At the moment, a key consideration of which plants are running and which are not is cost, but if links were broken, power could be supplied from closer but more expensive generating stations. Another possibility is to break up the grids into "islands" if critical links are severed.

Richard J. Lordan, a senior technical executive at the Electric Power Research Institute, a nonprofit utility consortium, said that one way the utilities could stop a wide-scale power failure was illustrated in the response to the San Jose attack: When utility system controllers detected the attack, they shut down some transformers that damaged cooling systems, so they would not overheat and fail catastrophically. If that had happened, restoration would have been much slower, he said.

A key problem for the utilities, Lordan said, was determining how much spending was prudent, in a system with "finite dollars and infinite wants." All costs will eventually be borne by consumers or taxpayers, he said.

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