The Food and Drug Administration has upgraded its classification of a broccoli recall initially reported in late December, labeling it as “Class I” — the agency’s most serious recall category.

The U.S. Food and Drug Administration has labeled the broccoli recall as a Class I, the highest risk level issued by the agency.

Bags of broccoli florets that were sold at Walmart stores in 20 states are affected by a Class I recall due to concerns over possible listeria contamination.