The grand jury, based in Fresno, Calif., is expected to begin calling witnesses next week. FBI spokesman Nick Rossi says the proceedings will be held behind closed doors.
Why convene a grand jury? According to CBS News Legal Correspondent Kristin Jeannette-Meyers, a grand jury is an incredibly powerful tool to aid police and prosecutors in their investigation. It is available in any case, but traditionally used in complex and/or high profile cases
First and most importantly, a grand jury has subpoena power. That means the grand jury has the right to call any relevant witness and require them to give testimony under oath.
This is particularly helpful in cases with reluctant or uncooperative witnesses. Police can't force a witness to answer questions. But a grand jury can. And if the witness "takes the Fifth," the grand jury simply grants immunity [unless the witness is a suspect]. At that point, the witness must answer or go to jail, which is what happened, for example, in the case of Susan McDougal in the Whitewater investigation.
In addition to witnesses, a grand jury can subpoena documents that police otherwise might have a hard time getting their hands on.
Another benefit: Grand jury testimony "memorializes" a witness' story under oath at a time when events are fresh in mind. This can be used to impeach a witness who tries to change his story at trial. Plus, there is great pressure on a witness to be absolutely truthful and completely accurate before the grand jury, because a lie could cause the witness to face criminal perjury charges.
One final note: Sometimes prosecutors prefer to have a grand jury indictment instead of simply filing the charges themselves in a high-profile or controversial case, because it helps to have the backing of more than 20 average citizens. In effect, they say to the prosecutors: "You are indicting the right guy."
In the case of the Yosemite murders, Jeannette-Meyers adds, a grand jury may be helpful in gaining the cooperation of witnesses who are frightened to give testimony. "That may be part of the scenario in this case," Jeannette-Meyers says.
Investigators may now know who killed the three sightseers, reports CBS News Correspondent Vince Gonzales. Justice Department sources tell CBS News that the FBI has three and possibly four suspects who are believed to be connected to the murders of Carole Sund, 44, her daughter, Julie, 15, and a family friend, Silvina Pelosso, 16.
The suspects are described by authorities as "sexual predators," known for stalking hotels and motels in the tourist area for victims. One female suspect was caught trying to use Carole Sund's credit card.
Sund's body and a second body, believed to be Plosso's, were found 40 miles northwest of the park in a burned-out rental car two weeks ago. Julie's body was the last of the three to be discovered, last week on a Yosemite hillside.
The three women were last seen alive Feb. 15 at a lodge in El Portal, after spending the day at Yosemite National Park.