(CNN) -- Police say it now appears that a southern California family missing since early February voluntarily left for Mexico.

But the apparent inactivity of bank cards, credit cards and cellphones since they vanished continues to baffle police and trouble family members.

"I've been in this business for over 35 years but never seen anything like this," Lt. Dennis Brugos of the San Diego Sheriff's Department told CNN.

"This whole thing doesn't add up," Michael McStay, the brother of Joseph McStay, said in a phone interview. "If they left, I don't need to know why. The bottom line is, I need to make sure they are OK."

Information gathered on the home computer of Joseph and Summer McStay indicates the couple was making travel inquires about Mexico on the Internet a week before they were reported missing, Brugos said.

"It had to do with travel in Mexico with children and whether you needed to have passports for children," Brugos said. "It looks as if there were some planning."

The McStays and their children, Gianni, 4, and Joseph, 3, have not been seen or heard from since February 4.

The McStays' two dogs were found unattended, without food or water, in the San Diego-area home, investigators said.

McStay's brother, who initially feared the family might have been kidnapped, still believes they are in danger.

"They can't fend for themselves," he said. "If something did happen in Mexico, I would not be able to live with myself."

A sport utility vehicle belonging to the McStays was found abandoned on February 8 two blocks from the San Ysidro border checkpoint into Mexico, authorities said.

Surveillance video taken that evening appears to show the family "casually" walking into Mexico, sheriff's investigators said.

Brugos believes the four individuals seen on the security footage bear a strong resemblance to the missing family, including the hats the children wore and the woman's boots and coat.

The sheriff's office met Tuesday with the department's liaison detectives in Mexico, he added.

"There is nothing to suggest any criminal involvement in their disappearance, but we would like to find the family," Brugos said.

Michael McStay, who said he will be handing out fliers at the San Ysirdo checkpoint and canvassing the stores, is desperate for any information.

"Who knows what happens once you leave U.S. soil?" he said. "We've all seen the news about what goes on down there. Even if they left voluntarily, were they going down for a weekend and never made it back? What would compel someone to go? I have no idea. If they left it would have had to be something heavy.

"It just doesn't make sense."

Joseph McStay is 5 feet, 9 inches tall and weighs 175 pounds. He has brown hair and brown eyes. Summer McStay, also known as Lisa Aranda-Martelli, is 5 feet, 5 inches tall and 115 pounds, with brown hair and brown eyes.

Anyone with information on the whereabouts of the McStay family is urged to contact Sgt. Dave Martinez of the San Diego County Sheriff's Department at 858-565-5200. Anonymous calls can be made to 888-580-8477.

A Web site, http://www.mcstayfamily.com/, has been set up in an effort to help locate the family.