That letter has come to be seen as a representation of the Russian military.
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And, some of those signs were seen by people who fled Russia's brutal invasion. Now, police are investigating and the city's Ukrainian community is responding.
"The community was invaded by hate and fear. Fear, with the letter 'Z,'" said Steve Demitro with the Ukrainian National Museum Board of Directors.
Representatives of Chicago's Ukrainian community banding together to denounce imagery. They say the letter "Z" Indicates support for Russia's invasion of Ukraine.
Surveillance video from the Ukrainian National Museum appears to show a man calmly placing the "Z" on the front door of the museum before appearing to take a picture and walking away. While Chicago police said detectives are investigating, ABC7 has blurred the man's face, because police have not said he's committed crime.
Father Pavlo Drozdyak fled Ukraine with his family in September. He said Ukrainian refugees have seen this "Z" on the side of Russian tanks firsthand.
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"For me, its symbol of death, you know?" Drozdyak said. "When you saw the symbol, they told us, 'we are here, and we come here to kill you.'"
ABC7 is told a Ukrainian veterinary practice is one of at least seven buildings targeted with the symbol.
The Ukrainian Village Veterinary Center has taken it down, but the owner, Danylo Butenko, said it's especially disturbing, considering the meaning of their mural.
"That mural is supposed to represent hope, unity and the beautiful culture of Ukraine. And, that 'Z' was placed right over of eye of the woman that was painted there," Butenko said. "To Ukrainians, that 'Z' is not just a letter. It represents genocide."
And, while many in the Ukrainian community feel the vandalism sends a threatening message, they have a message of their own.
"This is definitely a hate crime, aimed at an ethnic group, at Ukrainians," said Ukrainian Congress Committee of America Vice President Marta Farion. "We will not be intimidated."