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.

By blowing up substations or transmission lines with explosives or by firing projectiles at them from a distance, the report said, terrorists could cause cascading failures and damage parts that would take months to repair or replace. In the meantime, it warned, people could die from the cold or the excessive heat, and the economy could suffer hundreds of billions of dollars in damage.

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.