A sniper attack in April 2013 that disabled several transformers at a substation near San Jose, CA was “the most significant incident of domestic terrorism involving the grid that has ever occurred,” according to officials, and may have been a rehearsal for a broader wave of attacks designed to plunge the entire country into darkness, according to a report published Tuesday evening on the Wall Street Journal‘s website by Rebecca Smith.

The Journal reports that Former Federal Energy Regulatory Commission chair Jon Wellinghof, a George W. Bush administration appointee who stepped down in November after seven years, secretly briefed the White House, Congress, and federal agencies last year to warn them that the attack on the substation–for which no arrests have yet been made–was no prank, but a meticulously planned and ultimately successful assault.

Wellinghof is particularly concerned that the nation’s power grid may be vulnerable to a few such attacks at key locations, and that terrorism experts have been so focused on cybersecurity that they have neglected physical security. The FBI concluded that there was no terror group behind last year’s attack, but a experts say it was not “an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation.”

Terrorism experts have begun to worry about various forms of attacks on the nation’s electrical grid, including the development of electromagnetic pulse weapons, which would involve the detonation of a nuclear device high in the atmosphere such that there is little physical damage to structures or people, but the power transmission grid is destroyed. Attacks such as the one in California would be far simpler–and perhaps equally devastating.

Image: Google Maps. The substation is point “A.”