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Concerns about a sophisticated attack on a California power station prompted top Senate Democrats on Friday to ask officials about whether stronger federal standards are needed to protect the U.S. power grid.

The letter follows news reports earlier this week about a 2013 sniper attack on the electrical substation that lasted 52 minutes and knocked out 17 transformers.

The April 16, 2013, attack on the Pacific Gas & Electric’s Metcalf transmission substation had not been widely publicized until The Wall Street Journal report Wednesday. 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.