I'm in Ohio. I never really understood the process until I was in it. The way it works is that a person is arrested for something that is considered a felony. The case goes to a local court (misdemeanor court) and if the judge feels that it is out of his league, it is bound over to the grand jury. Then, as grand jurors, we hear the facts of the case only from the prosecutors side. That's where the police come in. They would swear in and then give testimony on what happened that led to the arrest and what the person was arrested for. Our job as the grand jurors was to decide whether or not there was enough evidence to have a trial. We didn't decide guilt or innocence just whether or not there was enough evidence for a trial.
It is very tricky not to vote to throw something out because you think the guy is innocent even if there is enough evidence to warrant a trial. There was a particular case that I had to vote to send it to trial, even though I was very passionate about the fact that the person was not only innocent but she was actually the victim. There were other cases that we threw out even though we thought the guy was guilty, there just wasn't enough evidence to go to trial. Either way, the police were at over 90% of the hearings. Usually it was the arresting officer but a couple of times it was someone in place of the officer because he was on vacation.