San Diego County sheriff's investigators say a missing California family's computer records show they had been looking into passport rules for children traveling to Mexico.

Homicide Lt. Dennis Brugos says investigators found e-mails indicating Joseph and Summer McStay were asking about entry into Mexico shortly before they disappeared.

The couple and their two young sons were last seen on Feb. 4. 

"It appears they went into Mexico of their own volition," the San Diego Union-Tribune quoted Brugos, who would not say to whom the e-mails were sent. "There is nothing to suggest any coercion or anything like that, at least that we have uncovered … this still isn’t a criminal investigation."

Last month, investigators said surveillance video may have captured images of the missing Fallbrook, Calif., family crossing the border on foot. The McStays' white Isuzu Trooper was found in a nearby parking lot.

Investigators have not been able to enhance the video enough to confirm the identities of the people in it.

However, relatives have called the disappearance out of character, especially since the couple left their dogs at home and food rotting in their kitchen.

Family members said the McStays did not give any indication they were going away, and are skeptical that the family is in Mexico. They added that Summer, 43, had openly expressed concerns about safety in Mexico, the Union-Tribune reported. She did not want to travel there with Gianni, 4, and Joseph Jr., 3, according to family.

Joseph McStay, 40, runs an indoor water fountain business, and his brother Mike told the Union-Tribune that nothing about the scenario makes sense to him. 

Detectives have found no evidence that the McStays had financial problems or were facing threats.

Click here to read more on this story from the San Diego Union-Tribune.

The Associated Press contributed to this report.