Businessman Joseph McStay, his wife, Summer, and their two young sons -- Gianni 4, Joseph Jr. 3 -- were last heard from Feb. 4, according to Sheriff's detective Troy Dugal.
Joseph's brother, Michael McStay, and Det. Dugal have been working for the last year to locate the missing family.
There were signs the family believed they were just leaving the house for a few short moments.
"There were some concerning things inside the house, where we are worried there might have been foul play," Dugal said. "We found eggs on the counter, popcorn left out."
None of which, according to Dugal, is normal for a family leaving for an extended period.
"There were pets in the backyard, so that's not very typical for a family who might have left for the weekend to go on a vacation," Dugal said. "We have not been able to absolutely confirm or deny absolute signs of foul play, so this remains open as a missing persons case,"
Investigators have concentrated parts of their search in Mexico, after finding the McStay's car near the Mexican border.
Detectives said surveillance video may have captured images of the Fallbrook family crossing the border on foot. At the bottom left corner of the video, a man can be seen walking hand-in-hand with a child. Following him is a woman in a white jacket, holding the hand of another child.
The video was recorded at the San Ysidro border crossing Feb. 8, the same day the McStay's white Isuzu Trooper was found illegally parked at a nearby strip mall.
Investigators also found e-mails indicating the couple had asked about passport requirements for traveling to Mexico with children shortly before they disappeared.
Relatives said it's uncharacteristic of the McStays to simply pick up and leave the country without telling family members.
Tracy Russell, Summer McStay's sister, said the idea that the family may have planned to go to Mexico has made her even more worried.
"I know I should feel a little bit better, but I don't," Russell said. "My mother is distraught. I don't think my sister would torture my mom like that."
She last spoke to her sister on the phone Feb. 4.
Michael McStay has set up www.mcstayfamily.com to field tips about the family's whereabouts. He forwards any promising information to investigators.
Anyone with information about the case is being urged to call deputies at (858) 974-2321 or (858) 565-5200 after-hours.
The show "Disappeared" airs on the Discovery Channel at 10:00 p.m.