The Sheriff's Office early in the afternoon declared the property a crime scene, but later clarified that it was still too soon to say whether the fire was set intentionally.
Deputies will patrol the area, which can't be entered without permission as they work to preserve the scene for the State Fire Marshal's investigation. Debris left by the fire is unstable, a news release said.
Cook County authorities say the fire started about 12:30 a.m. Tuesday. The lodge, built in 1952 on a resort site that dates to the late 19th century, was quickly engulfed. By later Tuesday morning, all that remained standing were two chimneys.
The lodge's general manager said no guests were checked in Monday night, and no one was injured in the fire.
The lodge was insured, owner Bryce Campbell said Wednesday, adding that he was "dreading the process" but intended to rebuild the lodge one day and reopen the resort to the public. He denied having anything to do with the fire.
Campbell, 39, bought the lodge in 2018 for $3.35 million, according to county records.
The State Fire Marshal released a statement Wednesday noting that the lodge, which had a working sprinkler system, had seven violations when last inspected in July; four of them had been repaired. Two of the outstanding violations involved the sprinkler and fire alarm systems and the need to provide recent inspection reports.
At this time, according to the statement, "It is too early in the investigation to determine if the three outstanding violations played a role in the fire."
A spokeswoman with the federal Bureau of Alcohol Tobacco and Firearms said a certified fire investigator with her agency has connected with state officials about the blaze.