Calvin Buchanon is a convicted criminal and former gang member.
"Stop the violence!" he bellows to a group of young toughs.
On Chicago's west side, Calvin is working his way toward a staff position with Ceasefire, an organization dedicated to reducing shootings and killings.
"Because I'm tired of seeing little kids get killed and dying out here for something that's not even worth it," he told us.
The problem is very real here. In the past year, 30 Chicago public school children have been killed on the streets, largely because of gang warfare. Those numbers represent a small portion of the violence that's plagued certain neighborhoods in the city. Yet, Ceasefire staff tell me, street violence would be worse if not for their efforts.
To demonstrate the point, Calvin and other Ceasefire members called "violence interrupters," showed me how they stopped a problem before it got out of control. They introduced me to members of the Vice Lords, a well-established gang that had recently been disrespected by another gang. Someone had smashed-out the window of their car.
"We were ready to kill someone," said Rio.
"You're gonna shoot a kid cause he kicks out your window?" I asked.
"I ain't gonna say necessarily kill him," answered Jim, another Vice Lord, "But you gonna mess somebody up."
I asked: "What do you get out of it if he's dead?"
"Relief," Jim answered. "No one else will kick out the windows thinking they're gonna get away with it."
Calvin is one of the messengers chosen to cool things down, someone with street credibility who's trying to prevent a murder from happening.
"They speak the same language," said Ceasefire's director, Gary Slutkin. "They use the same cuss words, and they can get into their minds."
Ceasefire's staff says the organization's approach to fighting violence is similar to how other organizations fight epidemics such as AIDS. The only way to stop the killing, they believe, is to send in credible messengers, like Calvin Buchanan, who can help relate to the people involved. In this case, Calvin's mission is to convince the Vice Lords that murder just isn't "worth it."
-- By Keith Oppenheim, CNN Correspondent