Nearly a year after a mysterious attack on a California power plant, officials are concerned — not just because the crime has not been solved, but because they’re afraid it may augur something worse.

“This is more about the larger issue of physical security of these high-voltage substations nationwide, and the need to ensure that some defensive measures are being put into place” former Federal Energy Regulatory Commission (FERC) head Jon Wellinghoff told CNN.

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As the Wall Street Journal reported on Wednesday, Wellinghoff was leading the commission at the time of the April 2013 sniper strike on a Pacific Gas & Electric plant near San Jose, California. He called it “the most significant incident of domestic terrorism involving the grid that has ever occurred.”

No arrests have been made in connection with the attack, during which the shooter or shooters knocked 17 giant transformers out of commission. More than 100 bullet casings consistent with assault rifle fire were recovered at the scene, but no fingerprints were left behind and the suspects left before authorities arrived. Underground telephone cables near the scene belonging to AT&T were also found cut. The company offered a $250,000 reward for anyone with information related to the attack, which has not been claimed.

Wellinghoff, now a partner at a San Francisco law firm, told the Journal that he convened a June 2013 meeting with utilities officials offering his own suggestions for them to increase security. He has also “made a habit” of visiting power substations to see how long it takes for security to question his presence.

“What keeps me awake at night is a physical attack that could take down the grid,” he was quoted as saying. “This is a huge problem.”

Watch CNN’s report on the attack, aired on Thursday, below.