More companies are recalling tens of thousands of packages of imported shrimp sold at Walmart, Kroger and other U.S. stores because they may contain radioactive contamination, according to federal notices.

AquaStar USA Corp. of Seattle is recalling more than 26,000 packages of refrigerated cocktail shrimp sold at Walmart stores in 27 states between July 31 and Aug. 16. The company is also recalling about 18,000 bags of Kroger-branded cooked, medium peeled, tail-off shrimp sold at stores in 17 states between July 24 and Aug. 11.

At the same time, H&N Group Inc., a wholesale seafood distributor in Vernon, California, is recalling more than 17,000 cases of frozen shrimp sold to grocery stores on the East Coast, according to a notice from the U.S. Food and Drug Administration. That recall began on Aug. 12.

The products have been pulled because they may be contaminated with Cesium-137, a radioactive isotope that is a byproduct of nuclear reactions. 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 a safety alert this month warning consumers not to eat certain frozen shrimp imported from PT. Bahari Makmur Sejati, an Indonesian company doing business as BMS Foods. Cesium-137 was detected in shipping containers from the company sent to U.S. ports and in a sample of frozen breaded shrimp.

FDA has also added PT. Bahari Makmur Sejati to a new import alert for chemical contamination to stop products from this firm from coming into the U.S.

It remains unclear how the containers or the shrimp became contaminated. Federal officials said they are investigating and declined to respond to detailed questions from The Associated Press about the source or extent of the contamination.