WASHINGTON (AP) — The Agriculture Department said Tuesday it had closed offices in five states after receiving anonymous threats that it considered serious.

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 an email to employees, Agriculture Secretary Tom Vilsack said USDA is closing the offices "due to the serious nature of these threats." He did not characterize the threats, but asked employees to be aware of their surroundings and report any suspicious activity. He said employees could telework or take authorized leave.

White House Spokesman Josh Earnest said the Department of Homeland Security is working with USDA "to ensure the safety of their offices and the personnel that work there."

The temporary closures may affect some tourists. In Colorado, the Forest Service's Canyon Lakes Ranger District tweeted that their information center is closed.