MIDDLETOWN, Pa. — The spirals of steam rising from the concrete cooling towers of the Three Mile Island nuclear power plant have defined the greater Harrisburg skyline for nearly a half century. On Sept. 20, they disappeared forever.

Three Mile Island, whose quartet of concrete towers represented the promise and peril of nuclear energy, closed 40 years after a partial meltdown forced more than 100,000 people to flee their homes in the worst nuclear accident in U.S. history.

The meltdown and subsequent radiation leak that started on March 28, 1979, led to sweeping changes in federal regulation of nuclear power, galvanized the national movement opposing nuclear power and drove a deep fissure through the riverfront communities that flanked the plant. For many, it was an economic lifeline, arriving just as the steel industry was fading; others saw a toxic time bomb in their backyards and vowed to fight until it was closed for good.

That day came Sept. 20, months after state lawmakers declined to approve last-ditch legislation to provide financial relief to the ailing nuclear power industry in Pennsylvania, amid the expansion of lower-cost natural gas and the historic availability of coal.

Nuclear energy has long provided the largest share of electric power generated in Pennsylvania — about 40 percent. But over the past decade, with the discovery of the Marcellus Shale deposit that courses below most of Pennsylvania, natural gas has surged ahead of coal and now produces almost 30 percent of the state’s power.

Critics, including environmental groups and the powerful natural gas lobby, condemned the legislation to save Three Mile Island as a $500 million bailout of a failing industry, similar to what New York, New Jersey, Illinois and Ohio have approved in recent years.

Lawmakers who back nuclear energy, including those with the four remaining plants in their districts, and the union representing many nuclear energy workers, argued the legislation, by adding nuclear power to the state’s Alternative Energy Portfolio Standards, would have helped address climate change, retained 600 jobs at TMI and preserved millions in local tax revenue.

Gov. Tom Wolf (D) supports nuclear power as an important part of Pennsylvania’s energy mix, said Wolf spokesman J.J. Abbott, but believes the legislation as drafted would have been too costly for consumers, especially those with lower incomes.

Now with aging reactors, lower cost alternatives and the specter of another accident, the decommissioning of TMI gets underway as the fate of Pennsylvania’s nuclear power production remains in limbo.

When it was commissioned in 1974, Three Mile Island was a source of pride in the Susquehanna River communities south of the state capital, such as Steelton, which bears the name of the industry that built it, and had been crippled by the loss of manufacturing jobs.

“We all embraced it,” said Eric Epstein, 59, chairman of TMI Alert , a community education and watchdog group, who grew up in Harrisburg. “It was wonderful technology that touched down in community where steel and coal took a hit in the ’70s. It meant jobs and money to the community.”

Everything changed on that Wednesday in late March in 1979.

Elizabethtown resident Pattie Longnecker and her husband, John, were on their way to a Lancaster County vineyard to gather vines for her wreath-making business when they stopped at a diner and heard on a trucker’s CB radio that there had been an accident at the power plant, three miles from their home. They turned around and sped home.

As hours turned to days of uncertainty about what exactly had happened, their concern turned to alarm and disbelief, Pattie Longnecker said. After all, government officials had, since the opening of Unit 1 in 1974, promised the plant was safe. When it became clear that there might be a radiation leak, the Longneckers fled to Virginia for a week.

The incident turned the former teacher into a community activist. She rallied her neighbors to fight to keep the entire plant closed. Later, as part of TMI Alert, she would educate residents and monitor actions of state and federal authorities charged with regulating the plant.

Unit 2, where the partial meltdown occurred, would close permanently, and Unit 1 remained closed for a year following the accident. When it reopened, citizens’ fear and distrust of government lingered and divided residents, many of whom were employed at the plant.

For communities near the plant, the question of latent health effects from the accident and the potential for future leaks remained a concern. Federal and state government studies over the years concluded residents experienced minimal radiation exposure, equivalent to getting a chest X-ray.

In 1996, a federal judge found there was insufficient evidence to link radiation from the accident to health issues cited among 2,000 plaintiffs. More recently, a 2017 Penn State study found a link between the accident and increased levels of thyroid cancer in the region around the plant, but the study did not conclude the radiation exposure caused the cancers.

Every August, in an eerie summer ritual, hundreds of residents within a 10-mile radius of TMI and the state’s four other nuclear power plants line up to receive free potassium iodide pills provided by the state Department of Health that protect the thyroid gland in the event of radioactive iodine exposure.

As the pills were being distributed this summer, the decades-long process of decommissioning was already underway.

In June, FirstEnergy, which owns Unit 2, began the process of transferring the long-shuttered reactor to EnergySolutions, which specializes in closing and decontaminating nuclear sites.

Exelon said the dismantling of the Unit 1 reactor and large components is scheduled for 2074 at a cost of $1.2 billion, which will be paid for by the decommissioning trust fund, provided by the utility.

Any additional costs will be covered by Exelon, the company said. Any radioactive low-level equipment will be decontaminated and taken to out-of-state disposal facilities, while spent fuel will be put into metal canisters and placed in massive concrete housings, known as dry casks, by the end of 2022.

“The storage facility is highly secured, and the casks are impervious to weather and risk of attack,” Exelon said in a statement. “The canisters emit very low radiation and they present no danger to the public.”

Critics, though, feel pushing the cleanup to the last quarter of the century could heighten the threat from radioactive waste and turn TMI into a nuclear waste ghost town.

Exelon Senior Vice President Kathleen L. Barron said while there are no plans to close its other facilities in Pennsylvania, the clock is ticking on nuclear power in the commonwealth and the United States.

No nuclear plants have been built in the United States since the Three Mile Island accident, and a number of those operating then have been or are being decommissioned, among them FirstEnergy’s Beaver Valley Nuclear Power Station near Pittsburgh, which is scheduled to close in 2021.

Since its reopening a year after the accident, TMI has operated without incident, providing power to 800,000 homes.

Across from the main entrance to the plant, an abandoned visitors center that once housed exhibits touting the future of nuclear power stands empty. Behind the center, rows of kale and sweet corn grow in a small vegetable garden, which is monitored for contamination. In a nearby grove of cherry trees, a plaque describes the trees as a gift from Japanese utility company researchers. The engineers had traveled to TMI in 1988 to learn about the accident in hopes of preventing a similar disaster in Japan. That was 23 years before the catastrophic accident at Japan’s Fukushima nuclear power plant.

For those who sought to keep TMI closed after the accident, the battle isn’t over.

Fears of flooding at the plant, leaks and the potential for terrorism are still real, Longnecker said. “It took over everyone’s psyche and has continued for 40 years. It’s still the cause of so much uncertainty, and it has cast a shadow over the whole area.”

“We will be gone,” said Longnecker, 75. “The bigger fight is the cleanup. It won’t be over until we are free of the contamination.”