Authorities on Sunday continued to search for a motive in the disappearance — and suspected killings — of an elderly couple who went missing from a nudist ranch in San Bernardino County.

Daniel Menard, 79, and his wife, Stephanie Menard, 73, were last seen leaving their home in Olive Dell Ranch and then reported missing Aug. 25 after they missed a church service, according to neighbors and police.

On Thursday, the Redlands police arrested a neighbor of theirs, 62-year-old Michael Royce Sparks, who was barricaded in a concrete bunker beneath his home when officers arrived. Sparks was arrested on suspicion of murder and booked into West Valley Detention Center.

Police said that Sparks wasn’t cooperative and surrendered to authorities only after “lengthy negotiations.”

Underneath the home, which is next to the Menards’, authorities say they found human remains that they are now in the process of identifying.

The Menards and their dog, a small shih tzu named Cuddles, were reported missing by a friend, police said.

The Menards’ car was found unlocked down the road from their home, and Stephanie Menard’s purse was found inside the home along with the couple’s cellphones, authorities said.

Redlands police spokesperson Carl Baker didn’t immediately respond to an email seeking updates about the case. He previously said at a news conference Friday that authorities were still “trying to locate the victims,” but that they were confident the Menards were “deceased and that they are still here on the property.”

Baker didn’t comment on a possible motive Friday. But resident Tammie Wilkerson, who lives next door to Sparks, told several local TV stations that Sparks hated the couple. The apparent feud between the Menards and Sparks involved a tree between their properties, Wilkerson told Fox 11, and Sparks “said it many times that he hated them.”

Other neighbors told the news station that Sparks was a quiet man who largely kept to himself.

According to its website, Olive Dell Ranch is a family- and couples-friendly nudist resort in Colton, Calif., that is an “ideal spot to enjoy the nudist/naturist lifestyle whether visiting for the day or an overnight stay.”