(CNN) -- Susan Blake writes her son Joseph all the time. "I send him e-mails," she says, "keeping him updated on what's going on."
Each e-mail goes unanswered. All she can do is hope that he sees them.
It's how she copes with one of the most unsettling and baffling missing persons cases in recent years: the mystery of the McStay family.
Next week marks the third anniversary of the disappearance of Joseph McStay, 43; his wife, Summer, 46; and their two young children, Gianni, 7, and Joseph Mateo, who would be 6 this month.
February 4, 2010 was the last day anyone saw the family or heard from them. When police entered their Southern California house more than 10 days later, they found eggs on the kitchen counter and bowls of popcorn in the living room. Their two dogs were also in the home. But not a single trace of the McStays.
Did they voluntarily leave? Are they in Mexico? Are they in danger? Are they dead?
Despite hundreds of tips and several high-profile television reports and interviews, no one has a clue.
"This is definitely an unusual case," says Lt. Glenn Giannantonio of the San Diego Sheriff's Department's homicide division "At this point we are no closer to finding them than on the day they disappeared. I know that sounds horrible, but we just don't know what happened to them."
The sentiment is shared by Dennis Brugos, who led the sheriff's department investigation into the McStays' disappearance before retiring last year. "Nothing makes any kind of rational sense that you can put together," Brugos says.
The McStays lived on a cul-de-sac in Fallbrook. Described on its website as the "friendly village," Fallbrook is about 18 miles from the Pacific Coast and some 50 miles north of San Diego.
Joseph McStay owned Earth Inspired Products, a company that built custom water features for high-end commercial businesses around the world, says Mike McStay, his younger brother. Summer stayed home to raise the children, but according to Mike was planning on getting back to work.
The family's locked and abandoned Isuzu Trooper was located February 8, 2010, in San Ysidro, just two blocks from the border. Surveillance video taken that night shows a family fitting the description of the McStays walking into Mexico.
"If you look at the clothing that they were wearing at the time, it coincides with when the vehicle was found; it leads you to believe that that was them crossing the border," Brugos says. "I can't say definitively it's them, but it's a high probability."
Giannantonio agrees. "We do believe the family on the tape going into Mexico is that of the McStays, " he says.
But Joseph's mother isn't convinced.
"My son's meds were in that car," she tells CNN. "My son has asthma and doesn't go anywhere without the meds on him. And to have babies go without car seats. This is the stuff that doesn't add up."
Mike McStay says he is not sure if the family is on the video. "The best is that the children appear to be size appropriate, but I could never get an ID off the adult. Even if it were them crossing, we still don't know to what intent. We don't know if it was under duress or willingly."
However, he says he believes someone has an answer. "How does somebody not see them?" he wonders. "They have to go get groceries, get the basic necessities of life; they have to have a way to generate an income. There has to be something.
"My brother is out there. I want what everyone wants, a phone call, a letter, something. I don't want to know why he left. We just have to know that they are OK."
Mike McStay is familiar with the theories speculating on what might have happened, including one presented in the new book, "No Goodbyes: The Mysterious Disappearance of the McStay Family."
Author Rick Baker says his own investigation, including reviewing hundreds of personal e-mails, shows that Summer and Joseph were having problems and that their relationship was, in his words, about to "explode."
Baker speculates, among other things, that at least one of the family members may have met with foul play.
Mike McStay bristles when asked to comment on Baker's suggestion. "I don't know how he sleeps at night," McStay says of the author. "I suspect he's looking for money. He's a good manipulator and knows how to twist things. He's just trying to sell books."
Blake, the mother, says there were many inaccuracies in the book. "It was very hard to read all this, very heartbreaking, the accusations."
She says, "The bottom line for me is this book will bring focus to finding the family, sad to say."
Giannantonio cautions there is no evidence of any criminal activity. "Everything leads us to believe the disappearance is voluntary. It's still categorized as a missing persons case," he says.
"We have to rely on facts and evidence. We don't want to publicize unsubstantiated theories. Everyone has theories, however we can't come out and say this is what actually happened unless we have something to back that up. Other people can, but that's not the business we are in."
Brugos says he thinks anything is possible. "If you are staying together as a group, as a family of four, it's probably a little more difficult. If you want the kids to go to school, questions are asked, and with the Internet everybody is an amateur detective."
Mike and his mother get reported sightings of the McStays on a regular basis. They come from all over the country. Mike, who says he chases them all down, recently received a tip from Belize, where his brother owned property.
"Every time one comes your heart pumps and you hope to God that this is it," Blake says, "and when it's not you fall apart. It's heartbreaking and very hard."
She isn't giving up. Neither is Mike, though both are mindful that ultimately the truth could be devastating.
"Until my dying day I will try to resolve this thing. I have to know where my brother and my family are," Mike says. "I'm going to have to stick with this for the rest of my life. Until we have closure. So that we can have some peace. I know all of us need peace."
Blake says the search for her family is the biggest fight of her life. "I refuse to give up. I hope to God, but at the same time it's been three years.
"How can a family of four just disappear?"