I was once told by a Federal law enforcement agent that they only indict people when its pretty much a slam dunk of getting a conviction. Of course the Clemens trial proved that wrong, but that could be explained away by relying on Andy Pettite, graduate of advanced studies from the Frank Pentangeli School of Witness Testimony. See training video below:
http://www.youtube.com/watch?v=90XT5GWlKaE