Updated

The U.S. could suffer a national blackout if attackers knocked out just nine of the country's electric-transmission substations on a hot summer day, according to a previously unreported study by the Federal Energy Regulatory Commission.

The study concluded that coordinated attacks in each of the nation's three separate electric systems could cause the entire power network to collapse, The Wall Street Journal reported Wednesday, citing people familiar with the agency's research.

The commission concluded that the three electrical systems that serve the entire nation could go dark if as few as nine of the country's 55,000 electric-transmission substations were knocked out in the East, the West and in Texas, according to the report.

"This would be an event of unprecedented proportions," Ross Baldick, a professor of electrical engineering at the University of Texas at Austin, told the newspaper.

According to Wall Street Journal, study's results have been known for months by the White House, Congress and officials at federal agencies, who were briefed by then-FERC Chairman Jon Wellinghoff and others at the commission.

Wellinghoff has voiced concerns about an April 2013 attack on Pacific Gas & Electric’s transmission substation in Metcalf, Calif., that lasted 52 minutes and knocked out 17 transformers.

The attack started when at least one person entered an underground vault to cut telephone cables, then attackers fired more than 100 shots, causing millions in damage. Workers were able to avert a blackout but it took them 27 days to repair the damage.

No arrests have been made in the case.

A memo prepared at the commission for Wellinghoff before he briefed senior officials last year said "the entire United States grid would be down for at least 18 months, probably longer" if attackers knocked out just nine substations.

The California attack "demonstrates that it does not require sophistication to do significant damage to the U.S. grid," according to the memo, which was written by Leonard Tao, FERC's director of external affairs, and reviewed by The Journal.

The memo suggested that an attack-induced blackout could be particularly long, in part because each of the three regional electric systems—the West, the East and Texas—have limited interconnections, limiting their ability to assist each other in the event of an emergency, The Journal reported.

Executives at several utility companies told the newspaper they are boosting security at their facilities. Virginia-based Dominion Resources Inc., said it planned to spend $300 million to $500 million within seven years to improve security capabilities.

Cheryl LaFleur, acting chairwoman of the FERC, said in a statement released late Wednesday that The Journal's publication of "sensitive material about the grid crosses the line from transparency to irresponsibility, and gives those who would do us harm a roadmap to achieve malicious designs."

"We take seriously our obligation to the American people to protect the reliability and security of our nation's energy infrastructure and to enhance its resilience," LaFleur said.

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