Whether to call a grand jury or not is within the discretion of the prosecutor. In this case I am sure you won't see a grand jury now. Grand juries are tools that allow prosecutors to charge defendants in a controlled and confidential setting without an opposition attorney being present. In this case we have two bodies and huge national interest with everybody expecting charges to be filed. The public sentiment and weight of evidence makes a grand jury rather superfluous at this point. We all know what is coming and the state has both parties under arrest and incarcerated. I believe they will take their time and, when ready, will file charges and ask for a preliminary hearing.
Sometimes if a prosecutor is unsure about a case he/she will use a grand jury to give himself/herself cover. If a grand jury refuses to indict the prosecutor can always say he/she tried. I testified in a police shooting case earlier this year in which the grand jury brought charges against one officer, but refused to hand down charges against another officer who had fired a shot at the same scene. Some people were critical the second officer wasn't charged and, indeed, it gave the defense a huge opening to tell the jury "officer X was not charged, so you must find the act of shooting was reasonable on the part of officer Y." In that case the use of a grand jury probably hurt the prosecution. I should note that it is VERY RARE for a grand jury to refuse to indict someone. Lawyers all know the aphorism "a good prosecutor could get a grand jury to indict a ham sandwich."