Contaminated metal at an industrial site in Indonesia may be the source of radioactive material that led to massive recalls of imported frozen shrimp, international nuclear safety officials say, as efforts are underway to halt more U.S.-bound shipments.

The International Atomic Energy Agency said Friday that officials are in “constant contact” with Indonesian nuclear regulators who have detected Cesium-137, a radioactive isotope, at a processing plant that sent millions of pounds of shrimp to the U.S.

“Preliminary information suggests that it may have originated from activities at a metal melting facility at the same industrial site or from the disposal of scrap metal junk to other areas of the site,” IAEA spokesperson Fredrik Dahl said in an email.

No U.S. investigators have been sent to the site in Serang, west of Jakarta, federal officials said.

Meanwhile, the company that exported the shrimp, PT Bahari Makmur Sejati, also known as BMS Foods, has recalled more than 300 shipping containers that were already on their way to the U.S., Dahl said.

The U.S. Food and Drug Administration warned last month that Cesium-137 was detected in shipping containers sent to four U.S. ports, and in a sample of imported frozen shrimp. That spurred multiple recalls of shrimp sold at Walmart, Kroger and other stores.

This week, additional recalls were issued by Tampa Maid Foods LLC, of Florida, for breaded butterfly shrimp sold under Admiral of the Fleet, Portico Seafood Classic and other labels.

◆ Stay up to date with similar stories by signing up to our WhatsApp channel.

None of the shrimp that triggered alerts or tested positive for Cesium-137 was released for sale, the FDA said. But other shipments sent to stores may have been manufactured under conditions that allowed the products to become contaminated, the agency said.

The risk appears to be small, but the shrimp could pose a “potential health concern” for people exposed to low levels of Cesium-137 over time, FDA officials said.

The FDA issued an import alert for shrimp from BMS Foods to stop the products from coming into the U.S.

The company sent about 12 million pounds of shrimp to U.S. ports in Los Angeles, Houston, Miami and Savannah, Georgia, in July and August, according to U.S. Customs and Border Protection records obtained by Import Genius, a trade data analysis company.

CPB officials alerted the FDA to potential radioactive contamination of multiple shipping containers. The National Nuclear Security Administration has sent emergency teams on “multiple deployments” to “isolate and characterize the extent of Cesium-137 contamination,” a U.S. Energy Department spokesperson said.

The level of Cesium-137 detected in the frozen shrimp was about 68 becquerels per kilogram, a measure of radioactivity. That is far below the FDA’s level of 1,200 becquerels per kilogram that could trigger the need for health protections.

Still, it is unusual to see that concentration of Cesium-137 in shrimp, said Steve Biegalski, a nuclear medicine expert at the Georgia Institute of Technology.

It’s possible that the contamination could have come from recycling old medical equipment that contained Cesium-137, Biegalski said. The material has been used in medical devices to reduce blood contamination and to treat cancer, for instance.

When such equipment is no longer useful, it can be recycled. But if the Cesium-137 isn’t properly removed, radioactive material can be released into the environment.

“If they get broken up in some sort of crushing mechanism, then all of a sudden, it’s basically a salt,” Biegalski said. “It would be like you taking a giant salt shaker and spreading it all over the kitchen floor.”

Containing the contamination is key, and it requires experts who have the training and expertise to respond, he said.

“It needs to be tracked down, isolated and cleaned up,” Biegalski said.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.