Detroit ? Two men were arrested in what law enforcement officials are calling a breakthrough in their investigation into a lethal form of heroin blamed for more than 100 deaths in the Detroit area.

Wayne County sheriff’s deputies and federal drug agents said the two suspects arrested Thursday sold a mix of heroin and the prescription drug fentanyl from the Jeffries housing project north of downtown.

Sheriff Warren Evans said investigators thought one of those arrested, a 45-year-old Detroit man, was “the key player who supplied the Jeffries project,” while the other was a lower-level dealer.

Fentanyl, a legally produced painkiller, is 80 times stronger than morphine. Officials in cities from Chicago to Philadelphia have reported a total of more than 200 deaths from the combination.