An expanded recall of frozen berries sold at Walmart and Trader Joe’s is underway over E. coli contamination fears, NBC News reported.

The recall covers frozen blueberries linked to an E. coli O145:H28 outbreak, according to the US Food and Drug Administration. The products have reached 30 states.

E. coli can cause severe stomach illness, with young children and the elderly at greatest risk. Symptoms include stomach cramps, diarrhea and vomiting.

Shoppers should check their freezers for the affected brands and lot numbers. Do not eat the recalled berries, officials warn.

Return the products to the store for a full refund. The FDA is still investigating the source of the contamination.

What to know: check labels carefully, throw out affected bags, wash hands and surfaces, and see a doctor if symptoms appear within days of eating the berries.

Frozen fruit has been linked to several outbreaks in recent years. The FDA advises cooking frozen berries when in doubt about safety.