Police released surveillance photos Saturday of the six teens believed to be involved in last Sunday's attack.
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According to police, the 15 and 16-year-old girls turned themselves in after seeing themselves on surveillance video.
The attack happened after 6:30 p.m. as 28-year-old Juatonna Davis, who is four weeks pregnant, and her friend got on a train at the Chicago Red Line stop.
Davis said she hasn't been to work the last couple of days because she's afraid to get back on the train.
"The whole time in my head I'm just praying that I make it home to my child," said Davis. "And it's scary because it's kids."
The teens allegedly admitted to robbing and assaulting two women on the train at the Roosevelt station, police said.
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The two girls were each charged with one felony count of robbery and two felony counts of aggravated battery in a public place, police said.
The incident comes after a string of violent attacks had been reported on the Red Line.
It's not clear if the incidents are related, but Chicago police say they're stepping up patrols near CTA Red Line stops.
Over the past year, over 1,300 crimes were reported on the Red Line, a nearly 12 percent increase from last year.
Red Line attacks:
2 beaten, robbed outside Red Line station in Loop Sunday, another man beaten and robbed earlier in day: police
Woman robbed of cell phone on CTA Red Line in Loop; 3 in custody
Man wanted for sexually assaulting, robbing woman on Red Line train: police