The U.S. Food and Drug Administration is warning people to avoid eating certain raw frozen shrimp sold in Walmart stores as the agency investigates possible contamination by Cesium-137, a radioactive isotope.

U.S. Customs and Border Protection alerted the FDA to the detection of Cesium-137 in a single shipment of imported frozen shrimp from the Indonesia-based company PT. Bahari Makmur Sejati, which does business under the name BMS Foods.

Officials have since detected the radioactive isotope at four ports across the nation: Los Angeles, Houston, Miami; and Savannah, Ga.

The amount of Cesium-137 detected within the contaminated shipment would not pose an acute hazard to consumers. However, long-term consumption would pose an elevated risk of cancer.

Bags of the frozen food product are sold under the brand name Great Value at Walmart stores in Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Missouri, Ohio, Oklahoma, Pennsylvania, Texas and West Virginia.

All containers and products that tested positive or alerted for Cesium-137 have been denied entry into the country, according to an FDA announcement on Tuesday. Federal officials are coordinating with Indonesian seafood regulatory authorities to investigate the root cause of the contamination.

There have not been any reports of contaminated shrimp in the freezer section or within the U.S. food supply chain, authorities said.

Cesium-137 can be found in soil, food and air, according to the Environmental Protection Agency. It is produced by nuclear fission for use in medical gauges and devices, but can also be a byproduct of nuclear reactors and weapons testing, the agency said.

Though bags of frozen shrimp haven’t tested positive for Cesium-137, officials say the product may have been prepared, packed or held in unsanitary conditions.

Walmart has received shipments of the frozen food that were imported after the date of the first detection, but those specific shipments did not alert for contamination, officials said.

Shoppers who purchased the following products should not consume eat or serve the frozen shrimp, but instead throw it away, authorities say:

• Great Value brand frozen raw shrimp, lot code: 8005540-1, Best by Date: 3/15/2027
• Great Value brand frozen raw shrimp, lot code: 8005538-1, Best by Date: 3/15/2027
• Great Value brand frozen raw shrimp, lot code: 8005539-1, Best by Date: 3/15/2027

The FDA is currently recommending that Walmart completely recall the item. BMS Foods is also no longer allowed to import the products into the U.S. until the company addresses any potential issues.