The posters advocate violence against anti-fascist groups and have been spotted in several locations including in Chapel Hill near UNC's campus, according to social media posts.
A black and white poster shared Tuesday on Facebook shows three figures holding weapons and reads: "Hospitalize your local antifa scumbag," referring to anti-fascist groups.
City leaders said signs on utility poles are illegal, and they are warning anyone who sees one, to report it.
"That's not something we want in the city of Durham," said Councilman Charlie Reece. "I think calls to hospitalize anyone are obviously of concern."
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Reece received several reported sightings of the fliers after tweeting a photo on his Twitter account.
If you’ve seen this poster in Durham (where this pic was taken this AM) or anywhere else in the country, please reply with a location. Thx pic.twitter.com/qku74GSMpP— Charlie Reece (@CharlieReece) August 22, 2017
Some responding they took them down before the posters reappeared.
"A known concern in other parts of the country is that folks who put them up also booby trap them with razor blades and other sharp objects, and so our staff has been warned and is taking due care in removing them," he said.
The fliers display the initials YWNRU with the email.
In a statement to ABC11, the company said: "While ProtonMail is a secure email service that is strongly in support of freedom of speech, our policy is that ProtonMail cannot be used for illegal activities. Inciting violence is certainly illegal and therefore against our stated terms of service."
The company would not confirm if the account had been suspended.
In an email, Durham's city planner directed staff to take precautions.
"Please have staff remove the referenced signage as a priority today, along with any other impermissible signage affixed to utility poles in the referenced vicinity," said Pat Young.
If you see the fliers, you are asked to contact Durham One Call online or call (919) 560-1200.