The grand jurors shall inquire into every offense triable within the county for which any person has been held to answer, if an indictment has not been found or an information filed in such case, and all other indictable offenses within the county which are presented to them by a public attorney or otherwise come to their knowledge. If a grand juror knows or has reason to believe that an indictable offense, triable within the county, has been committed, he or she shall declare such a fact to his or her fellow jurors who may begin an investigation. In such investigation the grand juror may be sworn as a witness.