It sounds like a science-fiction disaster: A nuclear weapon is detonated miles above the Earth's atmosphere and knocks out power from New York City to Chicago for weeks, maybe months.
Experts and lawmakers are increasingly warning that terrorists or enemy states could wage that exact type of attack, idling electricity grids and disrupting everything from communications networks to military defenses.
The Federal Energy Regulatory Commission is pushing Congress for authority to require power companies to take protective steps, which could include building metal shields around sensitive computer equipment.
An expert panel that Congress created to study such an attack says it would halt banking, transportation, food, water and emergency services and "might result in defeat of our military forces."
"The consequences would be catastrophic," said Joseph McClelland, director of the energy commission's Office of Electric Reliability.
"It would bring down the whole grid and cost between $1 trillion and $2 trillion" to repair, said Rep. Roscoe Bartlett, R-Md. Full recovery could take up to 10 years, he said.
The scenario involves a phenomenon called an "electromagnetic pulse," or EMP, which is essentially a huge energy wave strong enough to knock out systems that control electricity flow across the country.
A nuclear explosion 25 to 250 miles above the Earth's surface would be high enough that the blast wouldn't damage buildings or spread a lethal radioactive cloud. Even so, at that height, the pulse would fan out hundreds of miles.
The immediate effect would resemble a blackout. Although blackouts can be restored quickly, an EMP could damage or destroy power systems, leaving them inoperable for months or longer.
House Homeland Security Committee Chairman Bennie Thompson, D-Miss., is pushing a bill to give the energy commission broad authority.
At a committee hearing in July, Steve Naumann of energy giant Exelon said the authority should be limited to "true emergency situations."
The commission studying the threat says the U.S. is ill-prepared to prevent or recover from an EMP, a vulnerability could invite an attack.
"We are not well-protected at all," said Michael Frankel, who was executive director of the commission.