JOLIET, Ill. (AP) — The coroner in Will County says he’s found evidence of the presence of a toxic substance in fatal heroin overdoses that’s more potent than fentanyl.

Will County Coroner Patrick K. O’Neil says he found traces of W-18 in the system of a 32-year-old Lockport man who died of a heroin overdose in April. W-18 isn’t as common as fentanyl but police have said it is even more deadly. Fentanyl has been blamed for the recent increase in heroin-related deaths across the U.S.

W-18 hasn’t been found in neighboring DuPage County, where Coroner Richard A. Jorgensen tells The Naperville Sun he’s seeing an increase in designer fentanyls. Jorgensen says the county had eight deaths due to fentanyl last year and so far this year that number is at least 17.