And law enforcement sources say agents are confident some of those responsible are already behind bars for other crimes.
Numerous sources say the probe centers on a group including two half-brothers with a long history of weapons, drug and sex offenses. One of the men's girlfriend says she has testified before a federal grand jury in Fresno.
Both men are in custody, one for wounding a police officer after a traffic stop, the other on a parole violation.
FBI agent James Maddock won't confirm the various accounts but says his task force is making substantial progress.
In April, CBS News Correspondent Vince Gonzales reported that federal agents had zeroed in on a small group of friends, ex-cons, and crystal methamphetamine users who they believe may be responsible for carjacking and murdering Carole Sund, 44, her daughter, Julie, 15, and a family friend, Silvina Pelosso, 16. At the time, sources told CBS News the FBI was focusing on four individuals, including Michael Roy Larwick and his half-brother, Eugene Earl Dykes.
The three women were last seen alive Feb. 15 at a lodge in El Portal, after spending the day at Yosemite National Park.
Larwick came to the attention of investigators just days after the victims' rental car was found torched and abandoned near his boyhood home. That's when he allegedly shot a police officer who tried to pull him over and kept police at bay for 14 hours. Larwick eventually surrendered and was grilled by the FBI about the murders.
In a jailhouse interview with CBS Affiliate KJEO-TV in Fresno, Calif., Larwick denied killing anyone.
"I ain't got anything to do with this," he said. "I done a lot of things. I'm no angel. But you've got the wrong guy."
Larwick has a long criminal record, with convictions for assault, rape, and attempted manslaughter, as well as numerous arrests on gun and drug charges. He was even arrested once with a rocket launcher.
But Larwick is so sure he'll be cleared of the Yosemite murders that he took a polygraph test and offered investigators a DNA sample.
"I volunteered my DNA, you know," Larwick said. "I don't have no reason not to. I'm sorry about that family and what happened to those people. OK? That's not me."
Officially, the FBI would not confirm any suspects, saying that could harm the ongoing investigation. Agents said they weren't ready to charge anyone, and they were going to take their time presenting the case to a federal grand jury.
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 or high-profile cases.
First and most importantly, a grand jury has subpena 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's 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 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.