Reporting for this story was supported by the Fund for Investigative Journalism.

One sleepy Saturday morning in late August 1959, the federal Atomic Energy Commission issued a press release.

During an inspection of fuel elements on July 26 at the Sodium Reactor Experiment, operated for the Atomic Energy Commission at Santa Susana, California by Atomics International, a division of North American Aviation, Inc., a parted fuel element was observed.

The fuel element damage is not an indication of unsafe reactor conditions. No release of radioactive materials to the plant or its environs occurred and operating personnel were not exposed to harmful conditions.

In fact, there was a partial nuclear plant meltdown in the hills just 30 miles northwest of downtown Los Angeles.

The details of what happened at the Santa Susana Field Laboratory, including the venting of an unknown amount of radioactive gases, did not receive much media attention, and the facts of the accident weren't really known to the public for the better part of two decades.

Today, the environmental damage has yet to be fully addressed.

Decades of Polluting the Mountains

Santa Susana was founded in the mid-1940s at what was then the remote fringe of a largely rural San Fernando Valley. The laboratory developed and tested 10 nuclear reactors for the federal government and tested rocket engines for half a century.

The 1959 meltdown was just one mishap in decades of pollution left by atomic research, the open-air burning of toxic wastes and thousands of NASA rocket engine tests.

Space agency technicians used at least 800,000 gallons of the carcinogenic chemical compound trichloroethylene (TCE) as a degreasing agent, then poured it out on the ground, where it flowed into unlined ponds and percolated down to local aquifers, records show. The EPA estimates that half a million gallons of the substance remain in the soil and groundwater beneath the lab. Other contaminants from NASA’s activities include perchlorate, hydrazines, PCBs, dioxins and heavy metals, the EPA found.

At its height in the 1960s, the laboratory employed some 9,000 workers and carried out as many as eight rocket engine tests a day. People remember how the thundering roar of the rockets used to rattle windows in the rapidly growing suburbs nearby.

Today, the lab sits idle behind a security fence. The last nuclear experiment there concluded nearly 30 years ago, and the rocket engine testing wound up in 2006.

Giant rocket engine test stands still loom intact, but many of the lab’s industrial buildings have been leveled. Those remaining are slowly weathering. The rusting industrial wasteland seems incongruous today, a silent blot on a vista of chaparral and majestic sandstone bluffs. Still, nature is fighting back. In places, black sage, mule fat bush and yerba santa are starting to crowd the roads. Mountain lions, bobcats, coyotes and deer roam the grounds.

The Boeing Co. took over most of the 2,850-acre lab site when it acquired Aerojet Rocketdyne in 1996, and has pledged to preserve its portion of the land as open space.

But some neighbors and nuclear watchdog groups call that a public relations ploy meant to obscure the extent of the contamination. They fear that by getting Santa Susana designated as parkland, Boeing could avoid hundreds of millions of dollars in cleanup costs. Environmental remediation standards for such land are less stringent than they are for places where people live. They point out that even if nobody ever lives on the mountain site itself, the suburbs extend to just half a mile from the lab gates.

Critics are also wary of the lab’s two other landowners, the Department of Energy, or DOE, and NASA.  In half a century of polluting the mountains, neither agency has come up with a thorough cleanup plan.

A Moment of Hope Turns Sour

In late 2010, the California Department of Toxic Substances Control -- the state agency in charge of regulating California’s most polluted sites -- tried to get things moving. It signed agreements with DOE and NASA  that required the federal agencies to remove all radioactive and chemical contamination from federally controlled property at Santa Susana, restoring the land to the condition it was in before rocketry and nuclear experiments began.

Activists called the agreement a triumph for the environment and public health. They trusted state regulators’ promise to make Boeing follow suit.

Instead, the standards for Boeing’s portion of the cleanup have been weakened to match company wishes. And both federal agencies are questioning the extent of their commitments to restore the land. The cleanup, which was supposed to be finished by now, hasn’t even cleared the planning stage.

Some activists wonder whether the toxic threats in the land will ever be removed.

“The history of this has been that of callous disregard for public health and safety, essentially cutting every corner you can, having a cozy relationship with regulators that lets you bypass normal rules,” says longtime lab critic Dan Hirsch. He's president of the nuclear watchdog group Committee to Bridge the Gap and directs the Program on Environmental and Nuclear Policy at UC Santa Cruz.

Cancer Risk Studies Hotly Debated

Several studies have linked Santa Susana to increased cancer risks. However, scientists associated with the laboratory’s owners have questioned such findings, saying they make unwarranted assumptions about how much poison people actually are exposed to, or extrapolate from study populations that are too small.

The debate has gone on for decades.

In 1997, UCLA School of Public Health researchers found that field lab workers who were exposed to radiation at Santa Susana have an increased risk of dying of cancer. Researchers estimated the risks at six to eight times higher than those permitted under federal guidelines for long-term exposure to low-level radiation. News stories at the time quoted scholars hired by the laboratory to review the findings as questioning the study’s methodology.

Two years later, the School of Public Health scientists published research showing twice as many lung cancers in workers who faced a lot of exposure to hydrazine on the job, compared with workers who didn’t. The authors said they couldn’t prove hydrazine exposure was to blame, but they were confident some chemical or chemicals related to hydrazine or other aspects of rocket engine fueling was the source of the danger. Again, scholars under contract with the laboratory’s owners called the findings inconclusive.

In 2005, Boeing funded its own study, which offered sharply different findings. After examining more than 46,000 people who worked for six months or longer at Santa Susana and an affiliated research facility in Canoga Park, researchers found a cancer death rate lower than that of the general population. Further, the paper’s authors pointed out that "no cause of death was significantly elevated," even among those exposed to radiation and toxic chemicals.

This time, press accounts quoted a UCLA researcher who noted that Boeing had paid a private research company more than twice what the DOE paid for the UCLA studies, and who speculated that the Boeing report’s conclusions were tailored to meet the client’s wishes.

The debate resumed the following year, when another UCLA scholar published findings on how poisons appear to have migrated from Santa Susana. Researchers reported that, from the 1950s through the '70s, people living within 2 miles of the field laboratory could have been exposed to significant amounts of TCE, hydrazine and other contaminants.

This time, Boeing replied with a letter to the study’s lead author, with copies to federal, state and local legislators, saying he was overstating the health risks by consistently choosing the worst-case exposure scenario. Boeing asserted that many of the contaminants cited in the report probably dissipate as they migrate from the site.

In 2007, University of Michigan researchers found the incidence of thyroid, bladder and blood system cancers to be more than 60 percent higher for people living within 2 miles of the site than for those more than 5 miles away.

For all the scientific claims and counterclaims, Santa Susana neighbor Holly Huff knows what she sees in her own body and among her friends and relatives. She has leukemia.

“A good friend, I just found out has esophagus cancer," she says. "My brother’s friend he went to school with all his life, over at Chatsworth Lake, she died last summer from a brain tumor. All the older people who lived above me, to the east, they all died of some sort of cancer. I don’t know. Is it just because cancer is everywhere? I don’t think so. I do know it’s here.”

Local Governments Say Cleanup Plan Fails to Address Problem

The DOE estimates that honoring its pledge to remove all the pollution it caused could mean excavating up to 1.4 million cubic yards of contaminated dirt from the 130 acres of land it is responsible for. That’s more than three times the volume of the Rose Bowl -- and removing that much dirt could cost at least $468 million.

A DOE study published in January proposes leaving more than a third of the department's pollution in place. It includes the option of reducing the cleanup much more and cutting costs by up to 75 percent.

The Ventura County Board of Supervisors, the Los Angeles County Board of Supervisors and the Los Angeles City Council have all protested the study -- and this April they reiterated their criticism in a joint letter. The state Department of Toxic Substances Control also has declared that the proposed cleanup fails to meet DOE's 2010 commitment.

For its part, NASA has estimated that keeping its agreement to remove all its waste would mean digging out up to half a million cubic yards of dirt at a cost of $200 million. The agency’s inspector general has pointed out that that’s more than three times NASA’s annual cleanup budget for the entire country. NASA has yet to come up with a concrete plan to proceed.

The Boeing Co., which is responsible for more than three-quarters of the land, hasn’t published any cost analysis as detailed as the federal agencies have provided.

Whenever Boeing’s site program closure director, Dave Dassler, talks about cleanup standards, he stresses the environmental cost of removing too much dirt.

“It doesn’t make any sense at all to put this habitat at risk, and people use the term ‘moonscape,’ ” he says.

Dassler advocates what he calls a “suburban residential” cleanup standard for Boeing’s property. Environmental activists say that he’s distorting that regulatory category and that in fact the standards he’s pushing don’t meet residential requirements at all. Dassler insists that the standard is safe, especially since Santa Susana will eventually become a park where nobody lives and people visit only occasionally.

He estimates that the company could finish its share of the cleanup by removing no more than 400,000 cubic yards of dirt.

That’s less than either NASA or the DOE face on their much smaller portions of Santa Susana’s grounds.

Boeing has acknowledged in a court filing that requiring it to match NASA’s and the DOE’s cleanup standards could cost the company hundreds of millions of dollars. Any comparison of Boeing’s plan with what the federal agencies are facing makes clear that Boeing stands to save a lot more money than that.

The Santa Susana project director for the DOE, John Jones, also has questioned whether his department’s promise to clean up its nuclear waste needs to be as rigorous. He points out that Santa Susana is the only place where the DOE has agreed to such stringent cleanup rules.

How Clean Is Clean Enough?

In 2012, the federal Environmental Protection Agency completed a detailed radiation map.

Eight out of 10 times, when technicians found radiation, it was at concentrations low enough that it wasn’t considered a health threat that must be removed. But that still left nearly 300 hot spots where radioactive cesium-137 exceeded the safety threshold. Another 153 samples had strontium-90 at much higher concentrations than regulators considered safe.

Robert Dodge of Physicians for Social Responsibility called the pollution a grave public health risk. To the human body, strontium-90 looks like calcium.

“It’s taken up and deposited where calcium would be, which means teeth and bones, therefore causing problems down the road of bone cancer and leukemia," he says. And cesium can actually cause cancer in any organ of the body."

During an October 2015 tour of the Field Lab grounds, DOE Santa Susana Project Director John Jones discounted the threat.

“Although there are those who are very politically connected who are very good for talking about vast radiological contamination, they need to define what is ‘vast amounts,’ ” he said.

Jones described most of the hot spots identified in the survey as just slightly more radioactive than what the EPA had declared safe.

“I’m an engineer, so to me, the numbers don’t lie,” he said.

However, the numbers Jones cited to discredit fears of radiation poisoning were out of date. A year before the interview, the EPA had markedly tightened its national safety guidelines.

Jones has not responded to repeated requests to say whether he still considers the cleanup too rigorous.

To critic Dan Hirsch, the EPA's new stricter rules mean that “the agreement to clean up everything they can detect, that they created, becomes critically more important."

Neighborhood Groups in Conflict

Some neighborhood organizations are also pushing for a strict cleanup. The Rocketdyne Cleanup Coalition and the Santa Susana Field Laboratory Work Group meet regularly for briefings on developments regarding the laboratory. Members frequently testify before local and state lawmakers, urging them to enforce the terms of the 2010 federal agreements and to strengthen the terms that Boeing must meet.

Another group, Teens Against Toxins, formed at the local high school. Co-founder Devyn Gortner has long since graduated.

Seven years ago, when NASA and the DOE signed their agreements, Devyn felt sure her neighborhood would soon be free of pollution.

Today, “It feels like we’re back on square one,” she says.

Still, some of Santa Susana’s neighbors say the environmental activists worry about the wrong things.

The SSFL Community Advisory Group is against enforcing the DOE’s and NASA’s cleanup commitments, arguing that scouring away all pollution will scar the beautiful Simi Hills, damage pre-Colombian historic sites, endanger local wildlife and jeopardize the health of neighbors. They are concerned about trucks carrying hazardous debris through the surrounding suburbs.

Group member Abe Weitzberg is a former Santa Susana physicist who lives about 3 miles from the lab. He says he helped start the advisory group in part because Hirsch and his supporters kept shouting down skeptics at  meetings.

The advisory group has been asking Southern California neighborhood councils -- appointed boards that report community sentiment to the Los Angeles City Council -- to oppose the state’s cleanup agreements with NASA and the DOE.

“You should only clean up those materials that pose a risk to communities. The hydrocarbons from the truck traffic actually pose more of a risk than very small quantities of cesium or strontium or chemicals, up at a place remote to where you live,” Weitzberg says.

But not everyone agrees. Melissa Bumstead attended the West Hills Neighborhood Council one night early this month when it considered a resolution opposing the agreements.

"They voted overwhelmingly for a very limited cleanup option as I held up a picture of my daughter with no hair," she says. "I feel that denial is a very, very powerful force."

When Bumstead and her husband, Chad, bought their home in 2012, they thought they'd found a fantastic deal. Melissa had checked crime statistics and the performance of local schools. They were thrilled with the backyard pool.

Then in January 2014, they took their daughter, Gracie, to a hospital emergency room after she suddenly showed bruising all over her body. Gracie was diagnosed with a rare form of blood cancer.

As her child underwent two years of chemotherapy, Bumstead said she kept meeting other parents at the local hospital whose children suffered from rare illnesses.

She's made a map of pediatric cancers in her neighborhood. She acknowledges that her data are comprised of family accounts.

Still, "even with our rudimentary data, the numbers are alarming," Bumstead says.

Dan Hirsch believes the advisory group is a case of "Astroturfing," where a polluter sets up a fake grass-roots organization that promotes its agenda and opposes strict environmental regulation. Email records show that a Boeing public relations executive advised the group as it was being formed.

But advisory group member John Luker gets tired of such talk. He says he has yet to see any pollution readings that make him fear Santa Susana will poison him. Meanwhile, he says, the need for a park is clear.

“There are people who say I work for the Boeing Co., or they qualify that by saying I work for the goals and the ends of the Boeing Co.,” he says. "My answer to that is the Boeing Co. works for me. Preserve what we can, clean up what we must, and save for future generations the wonders that are up here.”

In August 2015, the DOE awarded a $34,100 grant to a foundation set up and run by advisory group members. Alec Uzemeck, a former advisory group chairman who now serves as the foundation’s secretary and treasurer, says the foundation was created for educational purposes.

“The $34,000 is for the CAG (community advisory group) to understand the cleanup process, the documentation, and what’s going on with the agencies and (polluters) and to communicate that to the public. So we’re a communications service to the public and from the public,” he says. “We don’t lobby.”

He declined to provide a copy of the bid request and grant documents.

So far, the DOE has proved equally guarded. Responding to a Freedom of Information Act request in October, a DOE official said he’d completed his review of the documents, but the department’s Washington office wanted to check them prior to release. The department has not produced the records.