For years, counter-terrorism officials have been fretting over how to best protect the United States from cyberattacks on the power grid—and rightly so. Just one well-placed strike on the wired network that delivers the nation’s electricity could potentially leave millions in the dark.

But while the technological approach to such threats is necessary, an analog attack could wreak just as much damage. That was demonstrated in April 2013, when a highly organized rifle assault on a San Jose substation nearly turned off the lights in California’s Silicon Valley.

“It is clear that the electric grid is not adequately protected from physical or cyber attacks,” Rep. Henry Waxman (D-Calif.) is quoted as saying in a December regulatory meeting. “[This was] an unprecedented and sophisticated attack on an electric grid substation with military-style weapons.... Under slightly different conditions, there could have been serious power outages or worse."

There wasn’t much of an outage because Pacific Gas & Electric was able to quickly reroute power from the disabled Metcalf power substation, according to the Los Angeles Times. But the minimal impact can probably be credited to luck rather than a lack of savvy by the perpetrators.

“These were not amateurs taking potshots,” Mark Johnson, former PG&E vice president for transmission operations, is quoted by Foreign Policy magazine as saying at a November event on power grid security. “My personal view is that this was a dress rehearsal [for future attacks].”

Although the Federal Bureau of Investigation has ruled out terrorism, little is known about the motivation for the attack or who might have been behind it. Although the incident wasn’t widely reported (perhaps because it occurred less than 24 hours after the Boston Marathon bombing) it has all the makings of a Hollywood thriller.

The blitz on the San Jose substation began about just before 2 a.m. on April 16, when at least two individuals accessed an underground fiber optics vault and cut cell and landline transmissions in the area, the San Jose Mercury News reported. With communications severed, at least one shooter fired nearly 120 rounds from a high-powered automatic rifle at transformers from about 40 feet away. They made almost every shot, the Times reported.

The 17 transformers—which were under fire for about 19 minutes—were targeted in such a way that they leaked oil and shut down rather than exploding and attracting attention. Piles of rocks near the substation fencing suggest that the gunmen had previously scouted the area for the best places to shoot.

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

The siege ended nearly an hour after it started, with police being dispatched after a traveler on nearby U.S. 101 reported seeing “fireworks” at the facility, the U.S. 101 reported seeing “fireworks” at the facility, the Times reports. The snipers fled leaving no evidence shortly before police arrived, leading authorities to speculate that they had also been monitoring radio traffic.

Perhaps more relevant than the who and why at this point is what can be done to mitigate physical attacks on facilities such as the San Jose substation. Jon Wellinghoff, Chairman of the Federal Energy Regulatory Commission at the time of the attacks, has been vocal about the need to meet low-tech fire with low-tech fire.

Something as simple as metal sheeting to block the site line to equipment could be effective, Wellinghoff said at the same meeting at which Mark Johnson spoke. He added that utility companies need more help developing security strategies and offseting the costs of implementing them.

“If you do all those things then you might be able to get them to move,” Wellinghoff said, according to Bloomberg News. “We have to start anticipating what will be our future with respect to both natural events and also events that may be perpetrated by those who would do us harm in the areas of physical and cybersecurity.”