A copy of one of the conversations was included in the notes and internal correspondence subpoenaed and turned over earlier this year. Assistant State's Attorney Celeste Stack said in court Wednesday the students used eavesdropping devices and wore hidden recorders.
It's illegal in Illinois to record someone without their knowledge or consent, unless there's a court order.
The material is related to the Medill Innocence Project and its attempts to exonerate Anthony McKinney in a three-decades-old murder case.
Stacks said prosecutors don't expect to file criminal charges because the statute of limitations expired.
Northwestern spokesman Alan Cubbage said the school didn't know about "any activities that may have violated state laws" and doesn't condone them.
Last month, a partner in a Chicago law firm withdrew from representing Northwestern University professor David Protess in the battle with Cook County prosecutors.