She was taken by ambulance by Illinois Masonic, where she was pronounced deceased.
The police believe they know who did it and are conducting a death investigation and a search for the attacker.
A very sad story. We will update as and when we learn more.
(Initial police reports gave the age of the victim as 13. We have since corrected it to 12.)
Update: The Crime in Wrigleyville blog has more details. The best news is that the suspect in the young lady's death is in custody.
Update: The murder took place at Chicago House at 4631 North Beacon. On its Facebook page, Chicago House says,
"Good morning supporters of CHASSA, There was a tragedy at our Families’ building last night. We are still discovering the details. We ask for your prayers and support for the family and our staff during this difficult time.
~ Chicago House and Social Service Agency"
Update: We can now post that the young lady's name is Alexis Stubbs, and she was a 5th-grader at Courtenay Elementary. The Tribune is reporting that the man suspected of killing her was her mother's former boyfriend, who was angered when he was asked to leave the apartment. We will post his name when charges are announced.
We realize that many readers are horror-stricken and furious about this senseless, inexplicable act of violence. Please remember that Alexis's family, friends and classmates are grieving, and give them respect and space to do so. They are dealing with every parent's worst nightmare. We understand that there will be a community vigil in the coming days, the timing of which will be chosen by Alexis's family.