A data point from FiveThirtyEight's coverage of Monday night's events in Ferguson is worth pulling out. "U.S. attorneys prosecuted 162,000 federal cases in 2010," the site's Ben Casselman writes, "the most recent year for which we have data. Grand juries declined to return an indictment in 11 of them."
As 538 and others have pointed out, this is a look at federal grand jury work, not state. Data for Missouri, where the Ferguson grand jury was assembled, isn't available. But it reinforces the go-to aphorism about grand juries: grand juries would return an indictment against a ham sandwich.
It's not always that simple.