WASHINGTON — Terrorists could black out large segments of the United States for weeks or months by attacking the power grid and damaging hard-to-replace components that are crucial to making it work, the National Academy of Sciences said in a report released Wednesday.

While the report is the most authoritative yet on the subject, the grid’s vulnerability has long been obvious to independent engineers and to the electric industry itself, which has intermittently tried, in collaboration with the Department of Homeland Security, to rehearse responses.

Of particular concern are giant custom-built transformers that increase the voltage of electricity to levels suited for bulk transmission and then reduce voltage for distribution to customers. Very few of those transformers are manufactured in the United States, and replacing them can take many months.

In a preparedness drill in March, technicians shipped three specially designed transformers from St. Louis to Houston and rapidly installed them in a marathon effort. The transformers were the electrical equivalent of a Swiss Army knife, with multiple attachments so that they can be used in a variety of jobs.

They are functioning well, said one of the experiment’s supervisors, Richard J. Lordan, a senior technical expert at the Electric Power Research Institute, a nonprofit consortium based in Palo Alto, Calif. But follow-up steps — like figuring out how many such transformers should be stockpiled as well as developing storage depots, financing purchases of the equipment and planning how to allocate it in an emergency — have yet to be taken.

Changes in the electric industry have made the grid more vulnerable in recent years, experts say. The grid was mostly built to serve the needs of individual utilities, but regulators have cut the generation companies loose from the companies that transport and distribute power to foster a competitive market. That has resulted in far more electricity being shipped much greater distances and in difficulty winning consensus to build new lines. Meanwhile, the Sept. 11 attacks and weather catastrophes like Hurricane Sandy have underlined the need for ever more vigilant monitoring and technological improvements.

“I don’t think we pay quite enough attention to the technology fixes that would allow us to make the power system more resilient,” said Clark W. Gellings, a researcher at the Electric Power Research Institute who is one of the report’s authors.

For example, the report broaches the development of submersible electric switches that could be operated after a hurricane. Some of the other technologies that have been suggested, like more sensors to help operators determine the status of transformers and transmission lines, would also help the grid on an average summer day.

The report urges that cheaper ways be found to put power lines underground, which would protect them from some effects of storms, and also calls for changes in infrastructure that would reduce the kind of mutual dependencies that result in wider blackouts. For example, more traffic lights could run on high-efficiency L.E.D. lamps and be equipped with batteries, and small generators could be placed in spots where power is needed for pumping water. The natural gas system could be equipped with pumps that run on natural gas instead of electricity so that the system would survive an extended blackout.

The notion of a looming attack on the grid has recently gained a conservative political following, with Newt Gingrich, who sought the 2012 Republican presidential nomination, championing a novel that imagines the crippling of the nation and the starvation of millions by unidentified enemies using high-tech methods to fry components of the grid with an electromagnetic pulse. The report does not discuss that possibility, but the appendix does include “electromagnetic pulse” among other technical terms.

The National Academy of Sciences report mainly refers to less sophisticated attacks but also warns of cyberattacks or infiltration of the grid’s transmission operators. “Even a few pernicious people in the wrong place are a potential source of vulnerability,” it said.

The report was completed in 2007, and after reviewing it, the Department of Homeland Security decided to classify its contents. The version released on Wednesday is redacted to avoid handing terrorists a “cookbook” on how to disrupt the grid, the report said.

Mr. Gellings, the researcher, said that despite the delay, most of the points it makes are still valid, although a chapter on cyberattacks is out of date.