They began to bend in the roaring wind, then their steel girders snapped like twigs, the towers toppled and the lights went out.

Minutes before the windstorm arrived to pummel the Washington area on June 29, it swept east through West Virginia, crushing three electrical transmission towers that are a tiny part of an intricate power grid that’s supposed to keep the lights on in America.

The term “grid” suggests a certain uniformity to the power system’s structure, but the network more closely resembles a patchwork quilt stitched together to cover a rapidly expanding nation.

The United States doesn’t yet face the critical shortage of power that has left more than 600 million people in India without electricity this week

But the U.S. grid is aging and stretched to capacity. More often the victim of decrepitude than the forces of nature, it is beginning to falter. Experts fear failures that caused blackouts in New York, Boston and San Diego may become more common as the voracious demand for power continues to grow. They say it will take a multibillion-dollar investment to avoid them.

“I like to think of our grid much like a water system, and basically all of our pipes are at full pressure now,” said Otto J. Lynch, vice president of Wisconsin-based Power Line Systems, “and if one of our pipes bursts and we have to shut off that line, that just increases the pressure on our remaining pipes until another one bursts, and next thing you know, we’re in a catastrophic run and we have to shut the whole water system down.”

India’s blackout was a power generation problem: It is saddled with aging coal power plants and facing resistance to new nuclear plants. This week, several plants closed suddenly and the lights went out. Although the United States will need more power plants to meet the demands of a growing population, the most immediate threat is that the delivery system will continue to fail.

The huge steel towers whose power cables crisscross the country — and the transmission stations they feed — are the pipes of that system. It’s not easy to store electricity for very long, and most of it is used within a second of being produced. At the push of a button, the grid routes power where it’s needed, from state to state or region to region. It is supposed to sidestep bottlenecks or hiccups that might slow the flow.

Towers are designed to withstand winds far stronger than the almost 70 mph blasts that struck Ritchie County, W.Va. But three towers in a row running parallel to Route 50 north of Ellenboro collapsed, early victims of a storm that would devastate power delivery throughout the Mid-Atlantic.

“A fourth tower didn’t come down but had to be removed because it was pulled off kilter,” said Todd Meyers, spokesman for FirstEnergy, a power company that supplies electricity to five states, including Maryland. “I don’t remember a time when this many came down. This is an unusual occurrence.”

Engineers are trying to figure out why the 40-year-old towers collapsed in a freak storm — whether through corrosion, foundation cracks or flying debris. But there have long been warnings that local systems, which began linking to one another in the 1920s, need an expensive overhaul.

“The aging of equipment explains some of the equipment failures that lead to intermittent failures in power quality and availability,” the American Society of Civil Engineers (ASCE) said in a report this year. “The capacity of equipment explains why there are some bottlenecks in the grid that can also lead to brownouts and occasional blackouts.”

The ASCE calculated that an additional investment of $107 billion was needed by 2020 to keep the electrical infrastructure whole.

“Electricity was primarily a luxury when the majority of our grid was built 50, 60 years ago,” Lynch said. “Most people didn’t require computers to do their jobs every day. They didn’t need the Internet access. IPhones didn’t need to be charged, and communication was all hard-wired, so you could still make a phone call when the electricity was out.”

Power customers in some parts of Manhattan experienced a brownout in July as Consolidated Edison reduced voltage to repair equipment and cut the systemwide load at the outset of a three-day heat wave.

Texas and Southern California are considered the most vulnerable this summer, but events have shown that a fuse blown almost anywhere can leave thousands of people in the dark.

A large part of downtown Boston and adjacent neighborhoods lost power in March when a connection between a power line and a transformer failed, shooting sparks that ignited mineral oil used as a cooling agent.

That led to a massive fire that consumed a substation, blacking out the financial and theater districts, emptying hotels and college dormitories, and shuttering restaurants, where food spoiled after two to three days without power.

The substation wreckage was too melted and twisted to determine why the connection failed. Two months later, while repairs were underway, the system short-circuited and shut down again.

“This was infrastructure that was not kept up,” said Glen E. Weisbrod, president of the Boston-based research group that helped produce the ASCE report. “There is no doubt that this is happening in cities all over. ”

Meg Mainzer-Cohen, who heads the Back Bay Association, remembers the first blackout as an economic disaster for businesses, which had to shut down for days.

“Back Bay went from a bustling, vibrant neighborhood into almost a ghost town,” she said. “The fire was in a place critical to the nervous system of the electrical grid. Now we’re advocating to develop ways to work around those sorts of issues. It’s really one of those penny-wise, pound-foolish investments not to make because the amount of revenues lost by businesses was astronomical.”

The grid’s fragility was demonstrated by a dramatic collapse that pulled the plug on Southern California in September.

It began in the Arizona desert just outside Yuma, where workers at a utility switch yard tried to revive malfunctioning equipment that regulates voltage flow. But something went wrong. Worried about the costly damage that could result from a sudden surge in power, the workers had to take down the faulty regulator, and, as a result, a 500-kilovolt line stopped transmitting power.

That caused an overload throughout the region that tripped an automatic shutdown switch more than 200 miles away at a nuclear power plant on the California shoreline.

Eleven minutes after the Yuma mishap, the lights and air conditioning went out in the middle of a heat wave for several million people in a swath that stretched from north of San Diego into Mexico.

Schools closed, planes were grounded for hours, traffic lights went dark and gridlock followed. People were trapped on rides and in elevators at SeaWorld and Legoland. Pumps failed at water-treatment plants, flooding San Diego Bay with more than 2 million gallons of raw sewage and forcing beaches along the coast to close.

What an investigating commission later called a “cascading and uncontrolled” shutdown became the most extensive power outage in California history.

Federal regulators said it was the worst grid failure since 2003, when a similar series of failures shut down power for 50 million people from New York City north to Canada and as far west as Ohio.

Among the four main causes for the Northeast blackout that investigators listed six months later: The primary utility “did not recognize or understand the deteriorating condition of its system.”

The utilities walk a fine line between satisfying their customers and keeping their investors happy, with costly expenditures in infrastructure bound to hurt profitability unless public utility commissions allow rates to keep pace with investment.

Regulators in Texas recently asked people to cut back on electricity use, saying the state’s power grid may have rolling blackouts this summer if there is a major heat wave or power generation drops.

Some of the private power companies have complained that Texans don’t pay enough for their electricity to support investment in a new generation of plants. The public utility commission responded with a rate increase this spring.

Pepco, which provides power to 788,000 customers in Washington and Maryland, has faced past criticism that it moved too slowly on infrastructure improvements. The widespread blackouts in the region after the June 29 windstorm led to renewed calls for burying power lines to make them invulnerable to falling trees and snapped power poles.

It would cost $5.8 billion to bury all of them in the District, according to an earlier report, an expense that Pepco said would add an average of $226 to the typical monthly bill over 10 years.

Failure to modernize the nation’s power system will have a price tag.

“By 2020, the cost of service interruptions will be $71 billion, or, if you break that down to households, $565 over that period,” Andrew W. Herrmann, president of ASCE, said

James J. Hoecker, chairman of the Federal Energy Regulatory Commission during the Clinton administration, warned that without significant fresh investment, “we’re headed for some serious financial and economic difficulties.”

“The investment gap that we’re facing is a little scary,” he said. “In fact, it’s a little more scary than the [ASCE] report indicates.”