Federal drug agents and the Centers for Disease Control and Prevention are stepping up the investigation of dozens of fatal overdoses linked to heroin laced with a strong prescription painkiller.

The synthetic painkiller fentanyl has tainted the illegal heroin supply in Illinois, Michigan, Ohio, Maryland, Delaware, Pennsylvania and New Jersey, authorities said. In Detroit, the drug combination has been linked to at least 23 deaths in less than a week, officials said Tuesday.

In Chicago two weeks ago, 15 people were hospitalized after using heroin bought on the city's West Side. All of the overdoses happened within a one-mile radius, authorities said. In all, dozens of people have overdosed in Chicago this year — including 10 who have died.

While officials are trying to figure out if a single source can be blamed, they say the fentanyl likely came from multiple sources.

"Samples are being tested to see if we can determine if it's pharmaceutical grade or from a clandestine laboratory," DEA agent Christopher Hoyt said.

At the request of Michigan health officials, two investigators from the CDC were sent to Detroit last week to look into more than 100 fentanyl-related deaths there since last fall, said CDC spokeswoman Bernadette Burden. She said it is the first time the agency has been called in to investigate fentanyl use.

At least 23 people have died in Wayne County, Detroit's home county, since Thursday in the latest string of deaths related to fentanyl, said Vanessa Denha-Garmo, a spokeswoman for County Executive Robert Ficano.

In southern New Jersey and the Philadelphia area last month, authorities said at least nine heroin users had died. Tests done in one community showed that what was sold as heroin was actually mostly fentanyl, with no heroin mixed in, they said.

Fentanyl is used to treat chronic pain and as anesthesia for open-heart surgery. The pain medication is 80 times stronger than morphine. Medical experts say fentanyl can cause breathing problems and make users feel as if they are suffocating.