A Chicago Police Department spokesman says the change will not affect felony-level narcotics cases.
Low-level, nonviolent drug offenses will not be prosecuted in Cook County during the coronavirus outbreak.
State's Attorney Kim Foxx on Friday announced the temporary policy as primarily a public health measure, but a letter from her top deputy shows the move resulted from a reduction in evidence testing at the Illinois State Police crime lab, which has trimmed on-site staffing due to the pandemic and curtailed its work on suspected narcotics.
In the letter, dated Thursday, First Assistant State's Attorney Joe Magats explained to a top Chicago Police Department official that, without the testing, prosecutors lack the evidence they need to prove up charges.
"ISP's directive that no narcotics will be tested has left us with no good-faith basis to proceed with these cases," Magats wrote to CPD First Deputy Supt. Anthony Riccio.
An ISP statement says its forensic science laboratories stopped testing "routine submissions" including suspected narcotics on Tuesday. The statement says tests were halted to "facilitate social distancing" by the labs' employees and says the labs continue to test evidence in violent crimes.
"We do not know how long these measures will be in place," the statement says.
CPD spokesman Anthony Guglielmi said in an email that the change will not affect felony-level narcotics cases.
"We have options to test evidence in federal labs or wait until our evidence can be tested at a later time," Guglielmi said.
Officials would not say how many narcotics cases had been dropped to date.
The testing change will have fewer immediate effects in Chicago's suburbs, where most police departments establish probable cause using field tests instead of the state police labs, according to Kane County State's Attorney Joe McMahon.
But McMahon said those field tests are insufficient as trial evidence.
"We may be in a situation where at trial we don't have results from a state chemist and we're unable to proceed," McMahon said Friday.
In Foxx's announcement, her office said halting prosecution of the low-level drug offenses was an effort to avert COVID-19 outbreaks in police and jail facilities and protect the health of detainees, cops, sheriff's personnel and the public.
"Everyone deserves to be protected, especially during these uncertain times, and we are obligated to ensure all members of our community feel safe, including those behind bars," she said.
Foxx's office, the statement adds, "is working collaboratively with the Cook County Sheriff's Office and the Public Defender to ensure any individuals who are not a threat to public safety are released from Cook County Jail."
Chip Mitchell reports out of WBEZ's West Side studio about criminal justice. Follow him at @ChipMitchell1.