Before the State of Illinois can formally charge someone with a felony one of two things must first happen: either a finding a probable cause by a preliminary hearing judge or a true bill of indictment from a grand jury.
What are the differences?
Preliminary hearings are adversarial, meaning both the prosecutor and defense attorney are present. These short hearings are usually for non-violent felonies, such drugs and firearm’s possession. The prosecutor typically calls to testify the arresting officer who answers a short list of questions: were they on duty on such and such a date at such and such location, did they see the defendant, what was the defendant doing, was there a search, what was found during the search. Cross-examination by the defense attorney is very limited. Questions that get into fourth amendment issues (illegal search and seizure) are not allowed. The burden on the government to get a case through a preliminary hearing is very low and so most defendants lose this hearing. What a defense attorney can do—especially if a future evidence suppression motion and hearing is likely—is get the officer committed under oath to a version of the events that cannot be changed in later litigation without risking impeachment.
Grand jury proceedings are secret, and typically reserved for more serious felonies, especially when someone has been physically harmed (murder, attempted murder, aggravated battery) or a person is the target of the crime (armed robbery, for example). Typically, if a detective got involved in the case, it goes to the grand jury. The defendant is usually not present, and neither is the defense attorney. There is also no judge. Grand jurors are selected from the voter pool much like other kinds of jurors. They usually serve for a set amount of time; a month, for instance. Present at these hearings are the jurors, the prosecutor, a court reporter and a witness. Grand juries have subpoena power and the jurors can ask questions of the witnesses, but I’ve seen this only once in over one-hundred grand jury transcripts. The witness is usually the detective who investigated the crime presently under grand jury consideration. Sometimes eyewitnesses are also called to testify. The questions are brief and similar to those asked in a preliminary hearing: were you assigned to investigate [insert crime], as the result of your investigation did you develop a suspect that was later identified as the defendant. It’s really that basic and these hearing transcripts rarely exceed ten pages in length. In fact, there’s a joke that it’s possible to indict a ham sandwich.
Federal grand juries are much different. The cases are usually more complex, and indictments are generally issued before an arrest and after a lengthy grand jury investigation; whereas in Illinois state courts, the indictment is usually issued after an arrest and following a very short hearing where there is little or no actual grand jury investigation.
Curiously, in Illinois, if a preliminary hearing judge finds no probable cause, the state can take the same case with the same set of facts and the same witness to the grand jury where almost always a true bill of indictment is issued. I have known several people to whom this happened. They were arrested and thrown in the county jail. About two weeks later at their preliminary hearing the judge found no probable cause, so they were set free that day; then a few weeks later they received a letter in the mail informing them they were indicted and have to appear in court…where bond is set again, and they go right back to jail. Happy they were not.