The arson attempt happened on Friday at about 11 p.m., according to the mayor.
Johnson said that security footage shows as the suspect started a fire outside the building before running off.
The fire did not spread past the initial flame, Johnson said. A Chicago police officer was on duty to monitor "the situation."
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"While we do not know the motives of this attempted arson, it goes without saying that this type of violence has no place in our politics," Johnson said during an unrelated press conference on Monday.
CPD said the blaze in the 100-block of North LaSalle Street caused damage to the building, but no one was injured.
No one is in custody, and CPD is investigating.
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