In October 2009, Joseph McStay excitedly e-mailed a friend about the house he and his wife had purchased in the San Diego suburb of Fallbrook, Calif. He boasted it had a "loft/game room & big yard for kids to go crazy," referring to their rambunctious preschoolers, Gianni and Joe Jr. "Love it."
So when the family seemingly disappeared four months later without telling any relatives, friends or business associates, and their car turned up four days later at the Mexican border, many people who knew them feared the worst.
"My fear is that I'm looking for two adult shallow graves and ... my two nephews' crosses," Joseph's kid brother, Michael, told the Orange County Register a few weeks after the Feb. 4, 2010, disappearance.
The mystery was largely treated as a missing-persons case until this month, after a motorcyclist riding through the desert above Victorville, Calif., came across a weathered skull and called police. Michael's worst fears were realized.
Two shallow graves contained the remains of Joseph, his wife Summer, 4-year-old son Gianni and 3-year-old son Joe Jr. Investigators say they have a lot of work ahead to find the killers, and no predictions as to who they are or why they would kill a whole family.
On Nov. 20, at a vigil where volunteers erected white crosses and released doves, Michael pondered aloud the same questions. "How did four people end up here in the middle of nowhere, over 100 miles from where they live?" he asked.
While no one has the answer, there are many leads, provided by people like Joseph's father, Patrick McStay, who has complained for years that the San Diego Sheriff's Department was not treating the case seriously, and Rick Baker, who traveled the world researching his book, No Goodbyes: The Mysterious Disappearance of the McStay Family.
“Investigators are questioning everybody," Michael McStay tells PEOPLE. "And they should be. We need to find the killer."
For more on the tragic case that has baffled authorities pick up this week's issue of PEOPLE, on newsstands Monday