It's common practice for prosecutors to employ their own detectives and interview witnesses for trial preparation.
If you want an official source..try the Cook County Investigations Bureau
The Investigations Bureau consists of more than 120 sworn officers who provide investigative and logistical support to Assistant State’s Attorneys in their preparation and presentation of cases. Investigators also complement and supplement local law enforcement efforts by providing them with investigative assistance, expertise and technical resources. Working with prosecutors in the Criminal Prosecutions and Special Prosecutions Bureau, Investigators also launch investigations of very specialized crimes that may not be handled by other law enforcement agencies, such as official misconduct, public integrity, election fraud, child support, and complex financial crimes.
Even the Law and Order franchise (which was also created by Dick Wolf the same as the Chicago-X franchise) had an, unfortunately short-lived, series featuring this preparation.
It was Law and Order: Trial by Jury
The series follows Bureau Chief Tracey Kibre (Bebe Neuwirth), an Executive A.D.A. assigned to Manhattan's homicide division. Kibre's team, including Detective Lennie Briscoe (Jerry Orbach) and A.D.A. Kelly Gaffney (Amy Carlson), follows up on leads and interview witnesses, as well as participating in trials, during which both sides examine witnesses and give arguments. Similarly, the defense's preparation varies from episode to episode, running the gamut from testing arguments in front of jury focus groups to deal-making between co-defendants. Several pretrial meetings are held where some procedural issue is argued and ruled on.