Heroin is folded inside of some notebook paper in this file photo. Authorities believe a toxic mix called "black shadow" could be causing a rise in overdose deaths.

Courtesy of the Washtenaw County Sheriff's Office

A toxic heroin mix called “black shadow” appears to be causing a rise in overdose deaths around southeast Michigan, according to a report in the Detroit Free Press.

Washtenaw County saw eight people overdose on heroin in two days last week and two people died as a result of those incidents. The Free Press reports the number of heroin-related calls in the region to the Poison Control Center in May nearly doubled from the same time period in 2012.

Acetyl fentanyl, an opiate derivative known to be cut with heroin that investigators believe is extremely dangerous, could be behind some of the deaths, according to the Free Press. Fentanyl first appeared in metro Detroit eight years ago and was responsible for more than 300 deaths, the report said.

• Read the full Free Press report here.

Kyle Feldscher covers cops and courts for AnnArbor.com. He can be reached at kylefeldscher@annarbor.com or you can follow him on Twitter.