Police said Jose G. Matias, 18, of Chicago had been arrested as of 3:15 p.m. Wednesday.
He was taken into custody without incident, police said. Charges are pending.
WATCH | Witnesses describe running for cover as shots fired inside mall
Rosemont outlet mall shooting sends shoppers running for cover
Police said the shooting appeared to be targeted between the gunman and Joel Valdes, 20, who was shot at the mall and died at the hospital.
Claudia Miller Negron can't make sense of her grandson's murder. The man, who she called JoJo, felt more like her son.
Alleged Rosemont mall shooter ID'd; victim remembered as 'lovable'
"He was with someone else. They were walking around, I guess shopping, and these people, whoever they were, I don't know because I don't recognize them as being a friend or anything, they were following them" she said. "They were leaving, and they confronted him and asked about a gang affiliation. He wasn't in a gang."
A 15-year-old girl, discovered inside a bathroom near the food court, was also shot in the wrist, police said. Her condition has since stabilized. Police said she was an innocent bystander who was not involved in the conflict between Matias and Valdes.
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Rosemont police said the shooter got away in a maroon 2008 Hyundai with others inside.
One firearm was recovered from the scene and is being processed as evidence, police said.
The sound of gunfire sent hundreds of shoppers scrambling for cover until police escorted patrons and employees store by store.
Investigators maintain the shooting was an isolated incident sparked by a fight between the alleged gunman and Valdes.
"His friends loved him. He was so lovable, he had a good heart," Negron said. "JoJo was always there for us."