Federal authorities are investigating several suspicious pieces of mail sent to election officials in at least eight states, some of which contained questionable substances and led to building evacuations.

Elections offices in Iowa, Kansas, Nebraska, Tennessee, Wyoming, Oklahoma, Missouri and Mississippi have confirmed receiving suspicious mail. The FBI and the U.S. Postal Service said in a joint statement Tuesday that they are investigating what they referred to as “a series of suspicious mailings sent to election officials in several states.”

“Some of the letters contained an unknown substance and we are working closely with our law enforcement partners to respond to each incident and safely collect the letters,” the statement said. “We are also working with our partners to determine how many letters were sent, the individual or individuals responsible for the letters, and the motive behind the letters.”

No state, so far, has reported that anyone has been harmed as a result of the suspicious mail.

The series of incidents is the latest disruption to election officials’ operations during an already contentious election season. Election Day is a little less than two months away, but early voting has begun in some states.

In November, suspicious letters mailed to election offices in at least five states forced evacuations and in some cases temporarily disrupted ballot counting after elections. At least one letter contained fentanyl.

On Tuesday morning, law enforcement was called to the Missouri Secretary of State’s Office to examine a suspicious envelope. JoDonn Chaney, a spokesperson for Missouri Secretary of State Jay Ashcroft (R), said the suspicious mail was identified in the office mailroom, and staff immediately contacted the Missouri Department of Public Safety. Chaney said Tuesday afternoon that the National Guard was removing the envelope.

The Mississippi Secretary of State’s Office also confirmed Tuesday that its elections division received a suspicious package the previous day.

“Thanks to information provided by federal agencies and partners, our office had been notified of the potential package and was on high alert,” the statement said. “Upon receipt, our staff handled the package in accordance with guidelines distributed by federal agents, and the package was promptly picked up by the Mississippi Department of Homeland Security where it is currently being tested.”

Several other states confirmed receiving similar suspicious mail earlier this week.

In Wyoming, the secretary of state’s office immediately contacted law enforcement and the office was evacuated after a white substance arrived in the mail.

“We take this very seriously,” Secretary of State Chuck Gray (R) said in a statement. “We will continue to work with law enforcement in monitoring this situation.”

An envelope identified Monday morning as suspicious by the Nebraska Secretary of State Elections Division “was isolated in a separate area in our office in accordance with established protocol,” according to a news release from Nebraska Secretary of State Bob Evnen (R). A substance in the envelope was identified, and it was later tested and found to be nonhazardous.

“The situation was quickly and safely resolved thanks to the swift action taken by our staff members, law enforcement and other first responders,” Evnen said.

A government building in Topeka, Kan., was evacuated Monday due to suspicious mail, the Kansas Secretary of State’s Office confirmed.

Iowa Secretary of State Paul Pate (R) confirmed that his office received a suspicious package Monday and immediately reported the incident.

“First responders reported to our office immediately and determined there was no threat to our staff or the public. We have now reopened our office and will continue to conduct normal business operations,” he said in a statement.

Tennessee’s Division of Elections “received a suspicious envelope containing a powder” Monday, according to Doug Kufner, communications director for Tennessee Secretary of State Tre Hargett (R). He added, “It was later determined that no threat to the staff or public existed, and the substance tested negative for hazardous materials.”

The Oklahoma State Election Board also received a suspicious envelope in the mail Monday which contained “a multi-page document and a white, powder substance,” Misha Mohr, a public information officer for the board, said in a statement. The substance was later identified as flour.