“In order to restore civilized society, the power has got to be back on,” said Scott Aaronson, who oversees the Electricity Subsector Coordinating Council (ESCC), an industry-government emergency response program.

While cybersecurity experts and industry executives describe such warnings as alarmist, intelligence officials say people underestimate how destructive a power outage can be.

The most damaging kind of attack, specialists say, would be carefully coordinated to strike multiple power stations.

If hackers were to knock out 100 strategically chosen generators in the Northeast, for example, the damaged power grid would quickly overload, causing a cascade of secondary outages across multiple states. While some areas could recover quickly, others might be without power for weeks.