A Halloween treat at the McHenry County courthouse turned into a Halloween trick when the building had to be evacuated Tuesday morning.

The McHenry County Sheriff’s Office confirmed the cause of a fire alarm Tuesday at the courthouse was a fog machine that had been brought into the building to mark the spooky holiday.

The Woodstock Fire-Rescue District responded to a fire alarm at the McHenry County Government Center shortly at 8 a.m.

“Firefighters quickly deemed the alarm was accidentally activated and normal operations were allowed to resume in the building shortly after,” said Alex Vucha, a communications specialist for the district, who confirmed that it was “Halloween decorations” that triggered the alarm.