Police think she was one of the people driving around the South Side Saturday near 79th Street and East End Avenue handing out free samples of the tainted heroin.
"I'm happy to report that as a result of that initiative, we have a female in police custody and we are formally classifying her as a person of interest," Johnson said.
There were several overdoses and people rushed to the hospital.
Activists like Andrew Holmes handed out flyers to alert people the drug was going around.
At the same time, Chicago police notified partners at the health department. They also sent in the organized crime narcotics unit to the Fourth District to do undercover work and find out where the tainted heroin was coming from.
Police said charges are pending against the person in custody. Three people remain hospitalized in serious condition.