For the latest on the McStay murders, watch "Buried Secrets: Who Murdered the McStay Family?" Tuesday, July 1, at 9 p.m. ET on CNN.
Victorville, California (CNN) -- Four years ago, Patrick McStay lost everything he loved.
His son, Joseph, his daughter-in-law, Summer, and their two little boys -- Gianni, 4, and Joseph Jr., 3 -- vanished.
"From Day One, I just had this gut feeling that I was never going to see them again," he said, swallowing tears. "I just knew. Something told me, I wasn't going to see them again."
The McStays disappeared from their home in suburban San Diego in February 2010.
There were no signs of a struggle. No apparent plan to flee.
Nearly four years later, the mother, father and two boys were found slain in the Mojave Desert -- their bodies buried in shallow graves.
How did they get there? Who killed them?
From the beginning, the case has baffled investigators, but they aren't giving up.
Said John McMahon, sheriff of San Bernardino County, in an exclusive interview with CNN: "It is certainly my hope that at some point in the future, we'll be able to solve this, and bring the suspect or suspects to justice."
February 4, 2010, began as an ordinary day in the McStay home in Fallbrook, a community of about 30,000 people about 18 miles from the Pacific Coast and 50 miles north of San Diego.
Patrick McStay spoke on the phone with his son, who ran a custom water feature business, and was scheduled to have a lunch meeting around noon. Summer McStay spent the day caring for the kids and overseeing the family's home renovation.
They were looking forward to their youngest son's birthday party that weekend.
But that night, the family of four suddenly left the house -- the doors locked, the car gone. Inexplicably, their two beloved dogs were left outside without food or water.
"(It's) as if you took off really fast but were coming back," said Susan Blake, Joseph McStay's mother, who is divorced from Patrick McStay.
"Your thoughts are going wild. 'Well, why would they be missing?' Something's not right here," she said.
Early evidence pointed investigators south.
Four days after the McStays disappeared, detectives say the family's white Isuzu Trooper was parked and subsequently towed from a parking lot just steps from the Mexican border.
And the car wasn't the only clue.
After they found the Isuzu, investigators discovered someone at the McStay home had done a computer search for getting passports to Mexico. They also found surveillance video showing a family of four matching the McStays' description crossing on foot into Mexico on February 8.
"I just thought, well, maybe they took off," said Joseph's mother.
But his father wasn't buying it.
"I said right up front, the first time I saw it (the surveillance footage), it wasn't them," said Patrick McStay, adding that Summer was afraid of Mexico.
"Would Summer take her two children in there? Heck, no," he said.
Patrick McStay worried detectives were chasing dead-end clues.
"I could have probably hired some Boy Scouts and done a better job," he said.
He reached out to Tim Miller, founder of the nonprofit search-and-rescue organization Texas Equusearch, which, in turn, contacted freelance investigative journalist Steph Watts for help.
One point that raised questions for Watts was the last known call from Joseph McStay's cell phone. The call was to a friend, Chase Merritt. It came in about 40 minutes after a neighbor's security camera captured the family's Isuzu pulling out of the McStay's cul-de-sac. Merritt didn't answer.
Among those questions, Watts said, were, "Did Joseph actually make that call from his phone, or did somebody else take Joseph's phone and make that call? Was he trying to call for help?"
The journalist also noted the impact of the delay in reporting the family missing to law enforcement.
Joseph's brother contacted authorities 11 days after the McStays disappeared. He says he waited because he didn't want to overreact, and thought the family might just be on vacation.
"The first few hours are so critical, the first few minutes ... The beginning of someone trying to commit a crime against you, that's the only chance you have to get out," Watts said.
The call the family feared finally came in November 2013, from an off-roading motorcyclist in the Mojave Desert.
More than 150 miles from the U.S.-Mexico border, and some 100 miles north of the McStay home, the biker found what looked to be part of a human skull in a remote area of Victorville, California.
Authorities investigated and found four skeletons in two shallow graves. With the help of dental records, they determined the bodies belonged to the McStays.
Once considered a missing persons case, the investigation moved to homicide. It also switched jurisdictions -- passing from the San Diego County Sheriff's Department to the San Bernardino County Sheriff's Department.
Jan Caldwell, with the San Diego department, defended her office's handling of the case.
"This is an incredibly thorough investigation," she said, her hand atop a thick stacks of files. "Thumbing through it, I can see phone records, I see photographs, I see communications.
"And to have done all of this -- to have compiled this kind of a massive file and still not know the answer -- enormously frustrating," Caldwell said last year, soon after the remains were discovered.
The San Diego County Sheriff's Department is no longer commenting on the case. It refers all questions to San Bernardino, which declines to get into specifics, citing the ongoing investigation.
Summer's mother, brother and sister also declined to comment to CNN.
Detectives still have not named any suspects or persons of interest.
"There was certainly evidence found in and around the grave sites, but at this point we're not prepared to talk about what evidence we did locate," said McMahon, the San Bernardino sheriff.
Watts said the only way the case will be cracked now is if someone talks.
"There was more than one person involved in this case because not one person dragged four people out to the desert and buried them single-handedly," he said.
"As the pieces begin to come together, it's looking to me like it was extremely orchestrated. So we have to ask ourselves, why?"
Like Watts, Patrick McStay believes the killer, or killers, has to be someone who hated his family for a reason -- but that reason is unclear.
So many theories. So many questions. So few answers.
"It's like a play. The first act has just ended. We've got three more acts to go," he said.
This story was reported by CNN's Randi Kaye and Jessica Small in Victorville and Melissa Dunst Lipman in New York. It was written by Dana Ford in Atlanta.