Senate Democrats are urging utility regulators to beef up security at power plants and substations around the nation amid an ongoing investigation into last April's sniper attack on a power grid station in California.
The military-style raid knocked out 17 transformers at a Pacific Gas & Electric Power substation, which feeds power to Silicon Valley.
"We are concerned that voluntary measures may not be sufficient to constitute a reasonable response to the risk of physical attack on the electricity system," the senators wrote in the letter to the Federal Energy Regulatory Commission and the North American Electric Reliability Corporation.
There are still no suspects or known motive, but FBI agents said they don't think it was the work of terrorists.
Meanwhile, lawmakers are calling the sophisticated attack a wake-up call on the threats to crucial infrastructure.
"This was an unprecedented and sophisticated attack on an electric grid substation with military-style weapons," Rep. Henry Waxman, D-Calif., said at a December hearing of the U.S. House Energy and Commerce Committee.
"Communications were disrupted," he explained. "The attack inflicted substantial damage. It took weeks to replace the damaged parts."
"The nation's critical infrastructure and defense installations simply cannot function without electricity," he said. "Yet it is clear that the electric grid is not adequately protected from physical or cyberattacks."