The Food and Drug Administration said Thursday that traces of the bird flu virus have been found in 1 in 5 samples of pasteurized milk, providing a more detailed picture of how much of the milk supply has been impacted.

The agency first said Tuesday that it had found viral fragments in commercially sold milk, triggering the U.S. Department of Agriculture to issue a federal order mandating that all dairy cows be tested for bird flu before being transported between states.

Health officials maintain — and experts agree — that pasteurized milk is safe to drink. The FDA detected small pieces of the virus in milk, not live, infectious virus. 

“To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the FDA said on Tuesday. The agency is conducting studies on the effects of pasteurization on the bird flu virus in milk and expects to release results in the coming days.

The FDA said it also plans to release additional information on the bird flu virus in milk supply later Thursday.

This is a developing story. Please check back for updates.