At issue in the investigation is when Jake suffered his fatal injury.
Two physicians at the hospital where Jake died concluded “it was an injury that occurred while he was in the care of the day care and not his parents,” according to a report by the Department of Human Services.
The pathologist who did the autopsy told a prosecutor the injury could have been days before death.
Edmond police hired an outside expert to try to determine the time of the injury. That expert's findings have not become public.
The grand jury is guided by assistants for Attorney General Scott Pruitt.
Grand jurors regularly meet two to three days a month at the attorney general's office across the street from the state Capitol.
Prosecutors from across the state often ask the attorney general's office for grand jury help with difficult cases, particularly when witnesses have refused to talk to police.
Grand jury sessions are closed to the public.
Oklahoma County prosecutors declined to comment Thursday.
Authorities also turned to the grand jury last year for help in the investigation of the 2010 beating death of Julie Mitchell.
She was killed in the master bedroom closet of her husband's $359,000 house in northwest Oklahoma City. Her killing has not been solved.