First of all that is not a statute you are quoting. Second, the standard is probable cause even under your quoted DOJ policy language - "probably be sufficient to obtain a conviction."
Finally, here's a typical federal grand jury instruction:
(5) The Evidence Needed Before a "'True Bill" May Be Voted
It is the responsibility of the grand jury to weigh the evidence presented to it in order to determine whether this evidence, usually without any explanation being offered by the accused, persuades it that there is probable cause to believe that a crime has been committed and that the accused was the person who committed it. Remember that the grand jury is not responsible for determining whether the accused is guilty beyond a reasonable doubt, but only whether there is sufficient evidence of probable cause to justify bringing the accused to trial. Only the evidence presented to the grand jury in the grand jury room may be considered in determining whether to vote an indictment.
HANDBOOK FOR FEDERAL GRAND JURORS
http://www.ndd.uscourts.gov/jury/jury_handbook_grand_jurors.pdf