More than 70 people across Chicago were treated for heroin overdoses within a span of three days last week, according to the Chicago Fire Department. Many of the overdoses occurred on the West Side, where more than 20 people had been treated in 24 hours and two alleged gang members were arrested for selling heroin to an undercover police officer, according to media reports.

On Oct. 2, it was reported that Chicago police had recovered a sample of the drug that could have been laced with the painkiller fentanyl at two West Side locations, including one in North Lawndale.

According to a report in the Chicago Tribune, by last Friday, Mount Sinai Hospital had treated 14 people who had overdosed on heroin, “some with the needles still stuck in their arm, according to hospital officials.”

Diane Hicks, a nurse and emergency room director at Mount Sinai, told the Tribune that the recent outbreak cold be similar to one that happened in 2006, when a rash of overdoses were attributed to fentanyl-laced heroin.

A statement put out by the National Council on Alcoholism and Drug Dependence notes that between 2005 and 2007, “more than 1,000 U.S. deaths were attributed to fentanyl, many of them in Chicago, Detroit and Philadelphia. The source of the drug was traced to a single lab in Mexico. The surge of deaths ended when the lab was identified and dismantled, the DEA said.”

As of Oct. 2, the Tribune reported, two of the suspected overdose patients at Mount Sinai’s “were in intensive care, another was still undergoing treatment, seven had been discharged and a few more were still in the emergency room as of about 3 p.m.,” Hicks said. Police were also looking into whether or not the apparent fatal overdose of a 49-year-old man in East Garfield Park could have been due to fentanyl-laced heroin.

Officials with the Drug Enforcement Agency (DEA), which is working with Chicago police to track the source of the possibly tainted heroin, say fentanyl is potentially lethal even in small doses. It’s used for a variety of medical purposes, including in cancer treatments and during painful surgery.

There have been recently reported outbreaks of tainted heroin all across the country, including in Ohio and Florida, where investigators “believe underground labs in China fueling the synthetics pipeline have concocted illegal Fentanyl as well as chemically tweaked ‘analogs’ typically sold as heroin or mixed with it,” according to a recent report in a local newspaper in the state.

Last weekend, as DEA and Chicago police officers roamed the West Side looking for the source of the tainted heroin, police arrested two men — Alfonzo Sylvester, 24, and Mario Wofford, 26 — who sold nearly 5 grams of heroin, valued at more than $700, to an undercover officer at the intersection of Congress and Independence Blvd.

It isn’t clear whether or not the heroin the men allegedly sold was laced with the possibly fatal fentanyl.

Courts records show the two men are members of the Unknown Vice Lords gang. Sylvester was arrested on Saturday and charged with two felonies — aggravated fleeing and eluding police, and possession of heroin with the intent to deliver.

Police arrested Wofford on Friday. He was charged with possession with intent to deliver and held on $10,000 bond since he had no prior violent criminal charges, according to the Tribune.