The avian influenza stalking wild and domestic bird flocks across the country has killed at least six endangered California condors since March and is suspected of killing another dozen.

The U.S. Fish and Wildlife Service said Wednesday it’s now attributing all condor deaths in the Arizona-Utah wild flying population to suspected cases of the virus. The deaths represent 15% of the region’s condor population.

Observers with The Peregrine Fund first saw a sick condor in northern Arizona on March 9, the U.S. Fish and Wildlife Service said. The condor was found dead under her nest 11 days later, and testing confirmed she was positive for the highly contagious flu.

Since then, 17 condors have died and five others are “receiving supporting care” and being tested, the wildlife service said.

The highly contagious avian flu, H5N1, has affected more than 58 million birds in domestic poultry populations and wild flocks across the U.S. and continues to spread, federal officials report.