USDA spokesman Matthew Herrick said the department had received "several anonymous messages" late Monday that raised concerns about the safety of USDA personnel and facilities. He said offices in six locations in the five states were closed Tuesday morning until further notice.

Herrick said the threat was one email message sent to multiple employees at all of the locations.

"Without getting into detail of the email message, USDA continues to work closely with federal and local law enforcement, including the FBI, to determine whether the threat is credible," Herrick said.

The closed facilities are in Fort Collins, Colorado; Hamden, Connecticut; Beltsville, Maryland; Raleigh, North Carolina; Kearneysville, West Virginia and Leetown, West Virginia.

They include offices for eight USDA agencies, including the Forest Service and the Food Safety and Inspection Service. Among the sites affected was USDA's sprawling agricultural research center and library in Beltsville, where employees were informed of the threat Tuesday morning and sent home. In Fort Collins, four buildings at the Natural Resources Research Center — a campus where over 1,000 people work — were closed.