The victim was trying to intervene in a violent argument among three people in front of the Foster Beach Apartments in Edgewater, police said.
Officers were in and out of the Wyndham apartments next door much of the day. A SWAT team also went in.
Police said there was an argument between a female suspect and male and female victim on the sidewalk in front of the apartment building in the 5200-block of North Sheridan Road just after 11:40 p.m. Wednesday, CPD said.
The suspect pulled out a knife and attacked the two victims. Police initially said the two victims were not injured, but later said a 26-year-old man was seriously hurt in the attack.
Another man, 21, tried to intervene and was stabbed in the chest, CPD said. He was taken to St. Francis hospital in Evanston, where he died.
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It was not immediately clear if he knew anyone involved in the incident.
Neighbors said they were investigating a woman who lives on the fourth floor, but CPD declined to comment.
Building management sent an email to residents at the Wyndham saying the tenant in question is in police custody. On Saturday, police said 23-year-old Isis Monae has been charged with the man's murder.
A neighbor of the victim, who did not want to share his name, said he was a nice guy. They frequently talked and played video games.
"I know he was probably trying to de-escalate the situation. It just didn't go well, I guess," he said.
The police presence has been quite shocking to residents.
"I think that's crazy; I mean, that's unbelievable. I can't believe she fatally stabbed somebody. That's insane," resident Tee Marcus said.
Marcus said he had numerous run-ins with the suspect.
"She would leave notes on my door calling me names," he said.
He said he complained to management and moved to a different apartment to get away from her.
The victim had not been identified Saturday morning. On Friday, two candles and a balloon now marked the spot where he was killed.
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The video in the player above is from a previous report.