"We received some tips," Roberts said, but would not elaborate on the information. He said charges against the pair could be filed Wednesday.
Roberts said the gun has not been recovered while authorities continue the investigation.
"We're not letting out all that we know," Roberts said.
The girl's northwest Indiana family had pleaded for help in the search for gunmen who shot the girl in the face Saturday night.
The shooting happened as the child sat in her stepfather's car in a Gary residential area.
Arbriana Turner was in critical but stable condition Sunday. Her parents said she still has a bullet lodged behind her eye. They also said doctors were waiting for swelling to go down before they made a decision to operate.
A reward had been offered in the case.
The girl's parents had been at Comer Children's Hospital in Chicago ever since the 12-year-old Indiana girl was flown there with a gunshot wound to the face.
At approximately 9:30 p.m. Saturday, while Ivan Woodard set out to take his stepdaughter to Walgreens, the first bullet was fired.
"The first bullet went sideways. The second one is the one that struck her and came through the car window. I ran around and grabbed her, and they just kept shooting," Woodward said.
"I was in the house. When I opened the door, there was blood everywhere. I didn't know what happened because I was in the house," said Loretta Tyler, the girl's mother.
Before getting into the car, however, Ivan Woodward heard two teenaged boys arguing with a neighbor.
"The young men came by to talk to my grandaughter, I said, 'She is too young. She is a minor,'" said a neighbor who identified herself as Vanessa.
"They were mad because there was a female next door that they were trying to talk to that didn't want to talk to them, and they got mad, and for whatever reason, just started shooting at us," he said.
The Sun-Times News Group contributed to this report.