Detectives said three of the four people shot are known gang members.
Police worked two crime scenes related to the homicide early Tuesday morning.
The shooting stemmed from a fight between two groups at the Shell station at the corner of West Belmont and North Sacramento avenues, CPD Chief Communications Officer Anthony Guglielmi tweeted later Tuesday morning.
This stems from a fight between two groups at the gas station. Both groups arrive in different cars, fight escalates to gun violence between each other & 4 are shot, 1 fatally. Dets reviewing private & city video. Anyone w/ info please email anonymously to https://t.co/yYAqzcVIfu https://t.co/GA8QiBe1h9— Anthony Guglielmi (@AJGuglielmi) May 22, 2018
Two men and two women in a Nissan Pathfinder, and one man and two women in a Honda Accord, arrived at the gas station around 2:45 a.m.
Police said two of the men got into a verbal argument, which turned into a fist fight when one man punched the other in the face. The fight then escalated to a shooting, after two people in the group pulled out handguns and opened fire, police said.
Surveillance video shows three people standing outside the gas station, throwing punches before guns are drawn.
"Investigation is still ongoing, but we do think they were shooting back-and-forth at each other," CPD Superintendent Eddie Johnson said.
Carlos Gonzalez, who lives nearby, was sleeping when he heard a series of gunshots.
"I was sleeping and I woke up and I heard shots, like six shots fired. I was like, 'What the heck?' But I was thinking it was construction or something," Gonzalez said.
Three men, ages 20, 25 and 37, were each shot several times. A 24-year-old woman was shot in the neck.
Police said the three people who were in the Honda drove a few blocks away as they called 911. They met an ambulance near West Belmont and North Albany avenues.
The 25-year-old man, identified as Omar Campuzano, was pronounced dead at Illinois Masonic Medical Center. The 37-year-old man was listed in serious condition at the same hospital. The 20-year-old man and the 24-year-old woman were both listed in critical condition at Stroger Hospital.
Police said several surveillance cameras captured video of the shooting, giving detectives a good idea of how it all unfolded.
Officers who gathered evidence at the Shell station found two handguns on the ground, which was littered with shell casings. There were bullet holes in the front door of the gas station.
"We don't know what to think. My children go to school right in the neighborhood and I've grown up here. My mother has grown up here. So I don't know what to think right now. It scares me," said Ellen Aceves, a neighbor.
"The neighborhood, it's a safe neighborhood. But lately it's just been getting a little crazy with gangs coming around from different neighborhoods," Gonzalez said.
"I'm hearing gun shots really early in the morning," said resident Alfredo Zamora. "One or two in the morning. It's not unusual to hear gunshots. Unfortunately, it's becoming trendy."
A woman who was in one of the cars, who was not injured, was taken into custody and was questioned by police. They are also looking to interview some of the other witnesses in the group, including two women who ran away after the shooting. Investigators are not looking for any other suspects.
Belmont was temporarily closed to traffic from Albany to Sacramento as police conducted their investigation. CTA buses on route 77 were temporarily rerouted in both directions near Belmont, Kedzie, Elston and California.