The toll has risen to 400,000 birds on 10 farms due to avian influenza in Indiana.
It started on a turkey farm and soon after the virus was detected on nine other farms.
Officials don't know whether the fall and winter waterfowl migration is to blame in Indiana.
They are also looking into workers traveling between farms, wind or other methods to learn how this new strain, H7N8, has spread.
Last year a different strain resulted in the loss of 48 million birds and disrupted the egg industry, mainly in Iowa, and the turkey industry, mainly in Minnesota and Wisconsin.
Migrating waterfowl were blamed for dropping dung with the virus to trigger those outbreaks.
In Indiana, control measures involved carbon dioxide pumped into barns to euthanize flocks which are being left in the barns for 30 days of composting.