What is the test for a grand jury to decide whether to indict a defendant in a sexual assault case?

The main function of a grand jury is to determine whether or not probable cause exists to believe that a crime has been committed, and, if so, to file charges against such persons as are reasonably believed to have committed it. Branzburg v. Hayes, 408 U.S. 665, 686-87, 92 S.Ct. 2646, 33 L.Ed.2d 626 (1972). An indictment, of course, is merely a charge and does not constitute any evidence of guilt. The grand jury has the equally important duty of protecting persons against unfounded or unsupported charges and, absent a finding of probable cause, it may not file an indictment. Id.