Investigators learned that an Internet search was performed on January 28, a week before the McStay family was last seen.
Joseph McStay's mother, Susan Blake, doesn't know what to make of it.
"It kind of shocked me that someone is inquiring about that kind of information because I know in my heart he would not take those babies over the border unless he was forced or drugged," said Blake.
But there is video of two children and two adults recorded at the San Ysidro border crossing into Mexico February 8. Investigators believe it's the McStays.
Investigators say the new discovery on the family computer found during a search for passport information only builds on their theory.
"We felt there was a possibility, a strong possibility, they left voluntarily and there was no coercion involved," said San Diego Sheriff's Lieutenant Dennis Brugos.
But Susan Blake still refuses to believe her son would just leave everything behind.
"They had a very good life. There was no reason at all to just up and 'poof' and disappear," said Blake.
Whether they left on their own or not, Blake just wants her family back.
"We need to find them," said Blake. "I worry about those babies. Are they fed, are they warm, are they happy, are they sad? My nights are hell, you know. You pray and cry yourself to sleep."
Authorities say the computer search was performed about a week before the family's Isuzu Trooper was found near the Mexican border with the kids' car seats still inside.
Detectives have found no evidence that the McStays had financial problems or were facing threats. Authorities are urging the McStay family to contact someone if they are safe and alive.