Highly pathogenic avian influenza hit two more barns in Iowa last week, leaving state and federal officials struggling to determine how the deadly disease made its way into the three barns in northwest Iowa.

"The equipment, the feed, the personnel, you have to look at all of those factors," said John Clifford, chief veterinary officer for the U.S. Department of Agriculture (USDA) and the Animal and Plant Health Inspection Service (APHIS) deputy...