The Woolsey Fire in Southern California sparked a panic last week after flames encroached on the site of one of America’s worst nuclear meltdowns, the Santa Susana Field Laboratory (SSFL) in Simi Valley.

The fire—one of three tearing through California—erupted last Thursday and has consumed more than 96,000 acres in Los Angeles and Ventura counties, claiming the lives of two people in Malibu. It was first reported at an intersection on SSFL property according to state fire protection agency Cal Fire and local TV station KTLA 5, and is now 47 percent contained. The site was once America’s sandbox for experimental aeronautics, weapons, and energy production—and the epicenter of a 1959 partial nuclear meltdown that was kept hidden from the public for decades. NBC called it “LA’s nuclear secret” in a 2015 investigative report.

California’s Department of Toxic Substances Control, which oversees cleanup of the former nuclear site, told Motherboard in an email on Tuesday that contaminated facilities “were not affected by the fire.” The agency said that measurements from SSFL and the surrounding community “showed no radiation levels above background levels, and no elevated levels of hazardous compounds other than those normally present after a wildfire.”

The Los Angeles County Department of Public Health found “no discernible” level of radiation in the area it tested.

But nuclear watchdog group Physicians for Social Responsibility disagrees, warning that noxious and radioactive matter likely spewed from the contaminated ground and into the air.

A “witch’s brew” of pollution—mercury, chromium-6, lead, and radioactive waste—is contained within the soil there, Dan Hirsch told Motherboard on Tuesday over the phone. Hirsch was director of the Environmental and Nuclear Policy Program at UC Santa Cruz and now leads the nuclear policy nonprofit Committee to Bridge the Gap, and worries about the Woolsey Fire’s potential release of these contaminants.

In a Tuesday statement, NASA confirmed “significant fire damage” across its portion of the site, which is jointly managed by the space agency as well as Boeing and the Department of Energy, but doesn’t foresee risks related to contamination. “Fire agencies responding to the site have determined the fire did not present any risks other than those normally present in a wildfire situation,” the agency said.

Hirsch described the zones managed by NASA and Boeing as being riddled with “exotic rocket fuels” from engine and component testing that lasted from the the Apollo era to the Space Shuttle era.

The portion of the site managed by the Department of Energy contains much of the radioactive contamination, Hirsch added, as this was the former site of the partial nuclear meltdown. The Department of Energy reported that its areas were not affected by the fire in a Tuesday statement.

“I would be concerned about the fire possibly causing radioactive contamination in the soil and plants to be lofted into the air and drift even farther away from the site, affecting even more people downwind,” Stephen Schwartz, nonresident senior fellow at the Bulletin of the Atomic Scientists, told Motherboard in an email. “It would not be difficult to monitor the smoke to assess the degree of danger.”

“The source and location of the fire is under investigation by Cal Fire,” a spokesperson for Boeing told Motherboard in an email. “It may have originated on or near Boeing’s Santa Susana site.”

Whether the point of ignition was within the boundaries of SSFL is still being investigated, a spokesperson for the Los Angeles County Fire Department told Motherboard on Tuesday over the phone.

Only moments before Cal Fire reported that the Woolsey Fire began, a “disturbance” triggered an outage of the Chatsworth substation belonging to the electric utility Southern California Edison. State regulators are now probing the possibility that the Woolsey Fire is connected to Southern California Edison infrastructure. The company said in a statement that it will “fully cooperate,” but there has been “no determination of origin or cause.”

Southern California Edison would not comment on the location of the Chatsworth substation, but a California Public Utilities Commission report from 2011 notes that it is located “within the larger Boeing Rocketdyne Santa Susana complex.”

Aerial footage posted to Twitter on Thursday by KCAL and KCBS photojournalist Stu Mundel shows the Thursday blaze emerging on what looks to be SSFL, near the substation and “a thousand yards from where the meltdown occurred,” according to Hirsch.

The legacy of SSFL is a painful one. Many Los Angelenos attribute the community’s unprecedented rates of cancer to the site. A 2007 study by the Centers for Disease Control and Prevention found that people living within two miles of the property had 60 percent higher rates of certain cancers.

Yet the cleanup process has long dragged out, with missed deadlines and claims that Boeing’s state-enforced standards for remediation aren’t strict enough.