It was identified at a state mail service center in Raleigh, according to officials.
Emergency crews responded and tested the substance. No one was injured.
State board staff are now wearing protective gloves when handling mail and taking additional precautions. The board also gave out training and guidance materials to the 100 county boards of election to ensure they are equipped to respond to the threat.
North Carolina is now the 20th state to get this kind of threat.
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"The goal here is to intimidate people who are engaged in counting the votes, engaged in administering our elections and it's strictly prohibited under the Voting Rights Act," said John Bonifaz, a Massachusetts-based constitutional attorney and co-founder of Free Speech for People. "When we threaten election officials, we undermine the entire process of our democracy and we undermine the right to vote."
Last November, similar envelopes were sent to offices in five states, four of which tested positive for fentanyl, according to the FBI.
"We ought to be focused on this," Bonifaz said. "It's a concern whether it's a benign or dangerous substance."
The FBI is leading the investigation into these new threats as well. The Bureau also released a statement in response to the threats:
"The FBI and U.S. Postal Inspection Service are investigating 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. 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. As this is an ongoing matter we will not be commenting further on the investigation, but the public can be assured safety is our top priority. We would also like to remind everyone to exercise care in handling mail, especially from unrecognized senders. If you see something suspicious, please contact law enforcement immediately."