The grand juries that decided not to indict officers Darren Wilson and Daniel Pantaleo are part of a system enshrined in the Constitution. The framers considered the grand jury an important check on government power.
But now there is growing criticism that government prosecutors can hijack the system to get results they want.
"The system is under the complete control, under the thumb, of prosecutors," said the Cato Institute's Timothy Lynch, who co-authored a 2003 scathing analysis: "A Grand Façade: How the Grand Jury was Captured by Government."
"If they want an indictment they are going to get an indictment," he told me. "If they don't want an indictment it won't happen."
A grand jury is significantly different from a regular jury in a trial. It meets in secret -- to protect those who may not be charged; prosecutors dictate what evidence and witnesses the jury sees. There's generally no judge or defense attorney.
Last week, St. Louis Prosecuting Attorney Robert McCullough defended the grand jury that declined to indict Officer Wilson.
"They were extremely engaged in the process, asking questions of every witness, requesting specific witnesses, requesting specific information and asking for certain physical evidence," said McCullough during a press conference.
But critics say grand juries rarely indict police officers for fatal incidents.
"Prosecutors are often unwilling to take on the police department because they work with the police week in and week out," explained Lynch.
There's another reason we rarely see grand juries indicting police officers - the law gives police a lot of discretion. If an officer reasonably believes a suspect is putting someone in serious danger, he generally can use deadly force.