The number of overdose cases on Chicago's West Side has jumped from more than 20 in 24 hours to more than 70 within 72 hours.
Chicago Fire Department spokesman Larry Langford said there has been 74 overdoses in the past 72 hours, noting that the number is more than double the rate of overdoses the city saw at this time last year.
Chicago authorities said Thursday they were investigating what's behind the spike.
A Chicago Department of Public Health alert obtained by the Tribune reported more than 20 people had overdosed in the neighborhood in just 24 hours.
Police recovered a sample that may have been heroin laced with the painkiller fentanyl purchased at two West Side locations. Langford said it was not clear how much fentanyl had been added to the heroin batches.
Chicago Police News Affairs on Friday said that 23 overdoses were reported in 24 hours, but no one had died as of Friday afternoon.
Police said multiple law enforcement agencies were working "to identify the source of the extremely strong batch of heroin."
