"It can take hours or sometimes even days, but given the size of this, it probably will be days," Normal Fire Captain Jim Vaughan said Friday.
He said town investigators completed their work before the insurance team's arrival. Although the fraternity intends to return to the site, it's unclear whether the project will be rehabilitation or a complete rebuild.
The multi-story house at 711 Kingsley Court, No. 2, is gutted. It sits along a circle drive next to another boarded-up fraternity house and across a concrete lot from other apartment buildings. The small street is just off South Kingsley Street and runs parallel to West Hovey Avenue.
Investigators may use bystanders' videos to help determine a cause and timeline. A damage estimate may not be available for a week, authorities said.
The property owner's name could not be confirmed.
Authorities earlier said arson is not suspected. Early reports suggest the attic fire may have been burning for some time before chapter members noticed a smoking ceiling fan in a second-floor bathroom.
Delta Sigma Phi members has 70 active members, and about one-third of them lived at the house; the 24 residents and seven guests escaped unharmed.
Illinois State University, other fraternities and sororities and the American Red Cross are helping the displaced students with housing, meals and other needs.