EL DORADO COUNTY (CBS13) — An arrest has been made in the deaths of a Benicia couple and their son in a burned-out Greenwood cabin.

Nolan Buchanan, 16, has been arrested in the deaths of his father, brother and his father’s fiancee.

Three sets of remains were found in the family cabin of Adam Buchanan, his fiancee Molly and their son Gavin. They haven’t been seen since Sept. 11.

El Dorado County Sheriff’s investigators say autopsies revealed the family had been shot before the cabin was set on fire.

RELATED: Three Bodies Found Inside El Dorado County Cabin Of Missing Benicia Family

Investigators say the teen had told investigators he had stayed behind, but they later found evidence to the contrary. They wold not go into specifics, citing the ongoing investigation.

Buchanan is the president and chief operating officer of a Benicia construction company. His neighbors at the Garden Valley cabin believe he and his family were there the day the fire started.

Nolan Buchanan had been staying with grandparents since the family disappeared.

A neighbor CBS13 spoke to saw Buchanan’s white truck returning home the night authorities say the fire started.

Police were seen searching Buchanan’s Benicia home, taking away computers and documents. Neighbors there reported seeing investigators towing away a white pickup truck.