Authorities have not said whether they believe Michael and Rebecca Hargon and their 4-year-old son, James Patrick, are still alive. Authorities reported finding blood and spent bullet casings at the family's home.
"At this time, the whereabouts of the Hargon family are still unknown," said State Highway Patrol spokesman Warren Strain.
Earnest Lee Hargon, an adopted cousin of the missing family, was charged with possession of methamphetamines and a firearm, Strain said.
"As a result of recent developments, that's all that we are going to charge him with today. We fully anticipate filing additional charges," Strain said.
Investigators also searched Hargon's property in southern Mississippi Saturday. Strain said possible evidence was found but did not elaborate.
Authorities closed off a county road on both sides of Leaf River Veterinary Services as part of the investigation. The clinic is on Hargon's property, and his wife works there as a veterinarian, Strain said.
The search area included the clinic, Earnest Hargon's residence and the banks of the nearby Leaf River, Strain said.