The U.S. Department of Agriculture, which closed offices in five states on Tuesday after receiving anonymous email threats, said all of its facilities would be reopened by Thursday.

The agency opened most of the offices on Wednesday after security was beefed up at the locations, department spokesman Matthew Herrick said in a statement.

The USDA continues to work with local and federal authorities, including the Federal Bureau of Investigation, to determine the seriousness of the threats, Herrick said.

The Forest Service and Natural Resources Conservation Service offices in Hamden, Connecticut, as well as the USDA office in Leetown, West Virginia, would reopen on Thursday, Herrick said.

He said the reopening of those offices was delayed because additional time was needed to get "security enhancements in place for a safe and secure opening or to make official notifications to union representatives."

Herrick added that he would have no further updates and referred questions on the investigation to the FBI. Carol Cratty, an FBI spokeswoman, declined to offer further information.

The closures were triggered by one email message sent to multiple employees at USDA offices in Hamden and Leetown, along with offices in Fort Collins, Colorado; Beltsville, Maryland; Raleigh, North Carolina; and Kearneysville, West Virginia, Herrick said on Tuesday.

In West Virginia, Jefferson County Sheriff Pete Dougherty told Reuters that his office was notified of the threat on Monday by a USDA employee in the Kearneysville facility in the northeastern part of the state.

"They were going to break into the building," Dougherty said.

He said the emailed threat did not name the facility specifically, and was similar to, or the same as, electronic threats sent to other facilities.

A number of universities were also threatened this week, school officials said.