Leanne See-Garcia's 12-year-old son found the skull near their driveway on Cherry Valley Road in Bull Valley on Tuesday.
"While he was waiting, there was a skull behind the mailbox. And he missed the bus that day so he came in so I could drive him and said he saw a skull and that it appeared to be a human skull," See-Garcia said. She moved the skull to prevent animals from taking it and called police. Responding officers found a second skull in the same area.
"I didn't know what to think. I didn't know if it was real. It appeared to be real but it looked older. I didn't know if it was science purposes or for classroom. I don't know why someone would throw a skull on the side of the road that way," See-Garcia said.
"The human skulls appear to be very old, so they're not fresh. They'd been there awhile," Kim Bostic, deputy coroner for McHenry County.
How the skulls got there and how long they have been there -- months or years, according to Bostic -- is under investigation. The Bull Valley Police Department and McHenry County Sheriff's Department searched for additional human remains and evidence on Wednesday, but have since called off those efforts.
"We've finished the search for today. Nothing else was found," Police Chief Richard Vance said. "The skulls that were recovered will be sent for someone to look at those, an anthropologist, so that should take a couple of days."
A forensic anthropologist may be able to determine the age of the skulls, their sex and possibly how they died. Officials say there is no reason for anyone in the community to be alarmed.
"There is no worry for the community," Bull Valley Chief Vance said. "We believe the remains found have no relation to any missing persons investigation."
Bull Valley is about 45 miles northwest of Chicago.