An unsolved attack on an electrical-power substation in California last year, in which vital transformers were riddled with gunfire and disabled for nearly four weeks, has left some experts worried that a chain of similar attacks could cause crippling damage to the U.S. power grid.

The Wall Street Journal provided new details Wednesday about an April 16 incident at the Metcalf transmission station in San Jose. The attack did not cause major power disruptions because officials were able to reroute electricity around the damaged substation.

The attack did not seem to be the work of amateurs. The shell casings left behind were devoid of fingerprints, and there were piles of small rocks near where the snipers took their shots that may have been placed by a scouting team, the Journal said.

“It’s still an ongoing investigation,” Brian Swanson, a spokesman for Pacific Gas and Electric, which owns the substation, said in a telephone interview with The Washington Post. “We’re not going to speculate as to the possible motive before the investigation is complete.”

The Journal’s account relied heavily on analysis from Jon Wellinghoff, a former chairman of the Federal Energy Regulatory Commission. Wellinghoff now works for a San Francisco law firm; he did not immediately return a call for comment.

A spokeswoman for the FERC declined to comment Wednesday.

These transformers are a vital, and difficult-to-replace, part of the country’s electrical grid. In essence, they help transfer electricity from the freeways of the grid — high-voltage transmission lines — into the electrical equivalent of side streets. Those are the lines that lead to homes and businesses.

In recent years, much of the concern about the safety of the electrical grid focused on cyberattacks. But the California incident demonstrated that a physical attack on a substation could also cause significant damage. For years, security experts have worried that if a number of these transformers were lost at once, the result could be a long-lasting breakdown in the grid.

In 2012, a report by the National Research Council found that high-voltage transformers “are the single most vulnerable component of the transmission and distribution system.”

“This has been the principal thing that has worried a lot of us, about vulnerability of the power system. Because transformers, especially the higher-voltage ones, tend to be very unique. They’re very hard to replace. And they’re very vulnerable,” M. Granger Morgan, a professor at Carnegie Mellon University who led the council’s study, said in a telephone interview.

“It’s probably a more serious issue than the cyber concerns,” Morgan said. “With cyber, it’s really hard to see that you could do much that caused long-term, serious damage. Whereas this is a strategy that could really mess up the power system not just for weeks, but for months at a time.”