The mother said the girl's uncle had told her about a video on Facebook fb Live that showed the assault.
The teens - a 14-year-old boy and a 15-year-old boy - face charges including aggravated criminal sexual assault and manufacturing and disseminating child pornography, police said in a statement.
"She's just having such a hard time even communicating what occurred to her", Deenihan said at police headquarters.
Judge Patricia Mendoza on Tuesday ordered a 15-year-old boy held in the Juvenile Temporary Detention Center while awaiting trial.
"She is truly traumatized but detectives are incredibly inspired and motivated by her strength and courage", he wrote, adding that "several young men" are still sought by police in the case.
The 15-year-old, who recorded video of the attack that he sent to a witness via Facebook Messenger, slapped the victim on the head while she performed oral sex on him.
Assistant public defender Liliana Dago said prosecutors hadn't proven probable cause for the 15-year-old, particularly on the child pornography charge.
Police spokesman Anthony Guglielmi said the girl was threatened by suspects, but he would not say what those threats entailed.
"The young men responsible, they should be ashamed of themselves", Chicago Police Superintendent Eddie Johnson said.
"We've seen a couple of acts in this city in the last few months involving social media, and it just disgusts me that people could watch these videos and not pick up the phone and dial 911", Johnson said. "It makes you wonder where are we going, what are we doing as a society?"
A second teenager is in custody in Chicago for an alleged sexual assault that was streamed live on Facebook. The girl's mother later told The Associated Press that the girl and the family were being taunted online and ridiculed and harassed by children in their neighborhood. After chatting with her for a while, the two boys made a decision to take the girl to a house located in West Side.
The girl's mother said in a brief interview Monday that her daughter remains at what police called the "safe place" and has not returned to school.
The investigation into the attack was fast-tracked because of a chance encounter the victim's mother had with Police Supt.
The teen was treated at a hospital after the assault before returning home to her family.