The girl has been identified to the Cook County Medical Examiner's Office as Esther Jung of Elk Grove Village.
Investigators said Esther 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.
The fort collapsed on both girls, killing the 12-year-old girl. She was in cardiac arrest when she was taken to a hospital and pronounced dead. The younger girl is being treated for hypothermia.
Arlington police called the incident an accident and did not suspect foul play.