WASHINGTON — The National Academy of Sciences warned in a study released Wednesday that a terrorist attack on the U.S. power grid could cost hundreds of billions of dollars and lead to thousands of deaths.

While such an event probably wouldn’t kill people immediately, it could cause widespread blackouts for weeks or months, according to a recently declassified report released Wednesday by the academy. If it occurred during extreme weather, heat stress or exposure to cold may lead to “hundreds or even thousands of deaths,” the authors of the study wrote.

While other entities have issued reports on electric-grid vulnerabilities, the study released Wednesday provides an unusually stark picture of what might happen if hackers, extremist groups, disgruntled employees or even energy companies sabotage the nation’s power network.

It calls for the government to create a national inventory of portable generation equipment that can be used during such an event.

An attack “could be carried out by knowledgeable attackers with little risk of detection or interdiction,” it said.

The study was sponsored by the Department of Homeland Security and completed by the National Research Council, which is part of the National Academy of Sciences.

Although the report was finished in 2007, the George W. Bush administration a year later prevented it from being distributed publicly. The panel of experts who prepared it pressed for its dissemination, contending it contained no classified information. In August, the Obama administration agreed to declassify most of the study.

National Academy of Sciences President Ralph Cicerone, and Charles Vest, president of the National Academy of Engineering, in a forward to the report, said its key findings remain “highly relevant.” The men also are chairman and vice chairman, respectively, of the National Research Council.

The U.S. electricity network consists of a complicated web of generators, high-voltage power lines, lower-voltage lines that run to homes and businesses, substations and other gear to keep electricity flowing smoothly across the country.

Since 2007, the increased use of computerized systems, including so-called smart meters — which give consumers greater control over their energy use — highlights the need for vigilance against cyber-attacks, David Owens, executive vice president of business operations at the Edison Electric Institute and a contributor to the study, said in a phone interview.

Threats to the network also include physical attacks on equipment that is often decades-old and lacks the technology to limit the effects of such an event, the study said.

While a hurricane or ice storm usually only takes down distribution lines that utility crews can put back up, terrorists can disable transformers, which may take years to replace, said Alan Crane, a senior scientist who worked on the report. A well-planned operation could take out several substations, he said.

“It’s the multiple attacks that have the really scary consequences,” Crane said. Although the probability of such a conspiracy is low, the consequences “could just be really awful.”

The report recommended Homeland Security take the lead in overseeing electric-grid security, working with the Energy Department and private companies to create a stockpile of mobile reserve equipment, including transformers, for the network. It also calls for the security agency to work with the Federal Energy Regulatory Commission, state regulators, utilities and grid operators to make sure they have “appropriate incentives” to upgrade the system.

Information for this article was contributed by Mark Chediak and Freeman Klopott of Bloomberg News.

Front Section, Pages 4 on 11/15/2012