Unspecified but credible threats have shut down the offices of the USDA in five states, according to an AP report.

USDA spokesman Matthew Herrick says in a statement on Tuesday that the department had received "several anonymous messages" that raised concerns about the safety of USDA personnel and facilities. He said six offices are closed until further notice.

Herrick said the department is working with the FBI and federal and local law enforcement to determine whether the threats are credible.

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 or say how they were received, but asked employees to be aware of their surroundings and report any suspicious activity.

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

The USDA oversees SNAP programs, agricultural development, the US Forest Service and the Natural Resources Conservation Service. It's unclear who is making the threats or who is targeted in them, but apparently they were serious enough to shutter six offices indefinitely.