The extra-alarm blaze erupted at 401 Washington Street in the vacant Toastmaster factory building, with the staging area at Washington and Harrison. The large, two-story industrial building has been abandoned about 10 years.
St. John's Lutheran School will be closed Monday due to the firefight.
Algonquin-Lake in the Hills Fire Protection District Battalion Chief John Greene said the building was fully engulfed.
"We're defensive on the fire. We're not putting anybody in it. There is no significant exposures next to it. The bike path next to it will probably be closed," Greene said. "Traffic will be difficult to get through today."
Greene said the cause of the fire was unknown and no injuries were reported. Greene said the roads around the fire would still be open to local traffic with the help of emergency personnel.
"Washington, Jefferson and the Railroad Street area will be difficult to get through, but with police assistance, they should be able to get local people through," Greene said.
The fire began around 4:30 a.m. and was called in by numerous passing motorists. Firefighters from nearby Cary were dispatched to assist about 4:50 a.m., according to a Cary firefighter.
As of 7:05 a.m., the fire was "mostly under control," according to Greene.
The Sun-Times Media Wire contributed to this report.