The McStay family vanished from its suburban Fallbrook home in February 2010 and may have gone into hiding in Mexico for reasons still unknown even though they apparently abandoned family pets and a six-figure bank account.
The San Diego County Sheriff's Department said in a written statement it appeared unlikely George and Summer McStay and their two young sons had met with foul play.
"The consensus is the family, in all likelihood, traveled into Mexico of their own free will," the statement said. "There has been no contact or communication from them since February of 2010."
The case took a turn early in the investigation when the family car was discovered near the San Ysidro border crossing. Video footage appeared to show the McStays entering Mexico with their two boys. They appeared to be traveling on alone and were not in any imminent danger.
The motivation for the family to pull up stakes and vanish was unknown. The mystery was profiled on Fox's "America's Most Wanted" and CBS' "48 Hours Mystery," KFMB-TV, San Diego, said.
Sheriff's detectives and FBI agents have been pouring over a large volume of tips. Since the sentiment was that the McStays and their children -- Gianni, 8, and Joseph, 6 -- had left the United States, the FBI should be the lead investigative agency.
"The FBI is the most appropriate law enforcement agency to continue the investigation since they have significant experience and investigative assets in foreign countries," the statement said.