She has been identified as Ceasia Ketchum.
Officials say she was swimming with her brother at Illinois Beach State Park last Friday when she went under.
Her brother tried to save her, but witnesses tell us she was under water for nearly 20 minutes.
When he couldn't find her, the family called 911, officials said. Soon, emergency crews took to boats and helicopters to find and pull that teenage swimmer out of the water.
"We saw the girl coming out, like, twice, and then we lost her and that's when I started panicking," said witness Silvia Mejia. "I saw someone screaming and I looked in that direction, and i saw this boy calling for help. So, I started screaming, like, 'Somebody's drowning! Somebody's drowning!'"
Christian Tortorello said he also swam out to help crews, using a life ring to help crews pull the girl to shore.
"I did what I could do with swimming out there. That's the best I could do because I'm not that good of a swimmer," he said.
"My prayers are with them, and then hopefully God brings peace to their hearts because I know I'm a mother, too, so I know how difficult it might be to go through this," Mejia said.
Authorities said the girl did regain her pulse while en route to Vista East Hospital in Waukegan, where she was in critical condition before being pronounced dead.