The girl has been identified to the medical examiner's office as Esther Jung of Elk Grove Village.
Investigators said Jung and a 9-year-old girl dug the fort Sunday in a snowbank near Rothem Church while their parents were at services inside the church. The adults found the collapsed fort about an hour later and called for help.
Police say the fort collapsed on both girls.
The 12-year-old girl was in cardiac arrest when she was taken to a hospital. She was pronounced dead a short time later.
The 9-year-old girl was treated for hypothermia.
On Sunday afternoon, police tape surrounded a huge mound of snow outside the church.
Arlington police called the incident an accident and did not suspect foul play.
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