Most of those drug sales occurred in areas of the West Side where homicides and shootings have been especially high this year, causing Chicago to see some of its worst violence since the 1990s.
"The people that are dealing this are street gang members, and they're dealing the drugs to fund the gangs and that's what puts guns on the street and what leads to all the violence," said Chief Anthony Riccio, head of the Police Department's Bureau of Organized Crime, which oversees the narcotics unit. "And it's so profitable that what we're seeing is gangs are willing to shoot and kill other gang members in order to take those spots over."
Riccio said 12 more suspects are being sought in connection with this latest drug investigation.
Drug dealers cut heroin with fentanyl to make a more potent product. The powerful, euphoria-inducing narcotic is 30 to 50 times more potent than heroin and significantly increases the risk of overdose deaths.
U.S. Attorney Zachary Fardon, whose office filed federal charges against three of the 33 suspects, noted that as little as 2 milligrams of fentanyl can be lethal.
Drug users often are oblivious to the presence of fentanyl in their heroin, according to Fardon.
"Many drug users don't know ... until it's too late," he said.
While the city's heroin supply is still largely smuggled in from Mexico, Dennis Wichern, special agent-in-charge of the Drug Enforcement Administration in Chicago, said the fentanyl being sold on city streets is coming from China.
"It either comes directly into the United States or it comes directly into Mexico," Wichern said.
Law enforcement authorities said much of the drug-dealing occurred near the Eisenhower Expressway, which has earned the moniker "Heroin Highway" because of drug users making the trek from the suburbs.
The Chicago police narcotics unit ran the investigation, using undercover officers to buy the drugs on the street in order to build cases against the 33 people arrested beginning Thursday.
Of those arrested, 28 are documented gang members, 25 are felons, 10 are on parole and seven have been arrested in the past with illegal guns, officials said. In addition, all but two of them are also part of the Police Department's "strategic subject list," a computer algorithm that found them at least 200 or 300 times more likely than the average person to be prone to violence, either as a victim or perpetrator.
Dozens of people have died over the past 18 months in Cook County of fentanyl-related overdoses.
Last fall, the Tribune first reported perhaps the Chicago area's most significant outbreak of fentanyl-related overdoses in about a decade as 74 people overdosed on the painkiller within a 72-hour period ending Oct. 1.
Chicago police at the time believed most of those victims bought the painkiller at one of several drug spots on the West Side.