Someone with knowledge of how a major electrical substation works sneaks into the sensitive area during the dead of night. First, he or she cuts fiber cables to knock out 911 and cell phone service. Then, he or she takes more than 100 shots from a high-powered rifle to "methodically" make transformers overheat and shut down, knocking out the substation. He or she then escapes, and is never caught.
This all happened last year during the night of April 16 at the Pacific Gas & Electric Company's Metcalf power substation southeast of San Jose, and according to a former PG&E official who talked to the Wall Street Journal, it could be a "dress rehearsal" for a major terrorist attack.
"These were not amateurs taking potshots," said Mark Johnson, a former PG&E official. "My personal view is that this was a dress rehearsal," he told the paper.
MORE: Surveillance Video Release From Sabotaged PG&E Substation
It may be far worse than that: Jon Wellinghoff, chairman of the Federal Energy Regulatory Commission at the time of the attack, calls it "the most significant incident of domestic terrorism involving the grid that has ever occurred" in the U.S., according to the Journal.
Michael McNerney, a security consultant for Delta Risk said the incident could be the latest example of how cyber terrorism could be used to attack America.
"Yes, this is a likely target, and something we're becoming more concerned about," McNerney said. "Critical infrastructure are things we rely on as a country in order to function: electric grid, air traffic control, water system, stock market. And if they go down, it could potentially be catastrophic."
The incident was first classified as vandalism by local authorities, but the investigation has since been taken over by the FBI, Foreign Policy magazine reported in December.
PG&E and the FBI have not officially called the April attack an act of terrorism.
The strange attack has grabbed attention of members of Congress. Retiring U.S. Rep. Harry Waxman said at a public hearing that it's proof that the country's electrical grid -- without which life as we know it would be powered off -- is not protected from attack.
Nobody lost power as a result of the substation attack after authorities rerouted power around it, but the substation was down for 27 days.
About a month later, sheriff's deputies spotted a "man in black" in a field not far away at 3 a.m. He also escaped.
So the attacker is still out there -- and now, people are worried.
NBC Bay Area's Scott Budman contributed to this report.