A couple living in a southern California nudist community who were reported missing earlier this week are presumed dead, police said on Friday, and a next-door neighbor had been arrested.

On Friday, police used a tank-like vehicle with a battering ram to smash into a home where they believed the bodies of Stephanie Menard, 73, and her husband, Daniel, 79, would be be found, said Carl Baker, spokesperson for the Redlands police department.

“We are confident that they are deceased and that they are still here on the property,” Baker said. He did not release any information about a possible motive.

Later on Friday, officials said searchers had found human remains in bags in a concrete bunker underneath the home. The remains were not immediately identified.

Michael Royce Sparks, 62, was booked into jail on suspicion of homicide on Thursday night after being found under the house at Olive Dell Ranch, which is described as a residential RV park and family-friendly nudist resort on its website. The rustic community is in hill country about 60 miles (97km) east of Los Angeles.

Baker did not know whether Sparks has a lawyer who can speak on his behalf. Online jail records showed he was ineligible for bail and was scheduled for a Tuesday court appearance.

A dog trained to find cadavers indicated the presence of at least one body somewhere under the house on Friday morning, but officers were waiting to enter until a structural assessment determined whether the home was safe, Baker said at a news conference.

“We’re not going to send people in if there’s a danger of the structure collapsing on them,” Baker said, noting that the assessment might take hours to complete.

A friend reported the couple missing on Sunday and said they were last seen at their home Saturday morning. Their pet dog, a shih-tzu named Cuddles, was also missing.

The couple’s unlocked car was found later on Sunday, not far from their residence, and both their phones, along with Stephanie Menard’s purse, were still inside their home. Daniel Menard was described as a diabetic with dementia.

Tammie Wilkerson, a friend of the couple, told KABC-TV that the Menards were kind people involved in the community.

“They’re very sweet people,” Wilkerson said. “There’s not a mean bone in their body at all, which makes this very confusing.”

The arrest followed rapid developments that began when police were at the park on Thursday to continue the investigation, search the area and knock on doors.

“We received information from a source that a person who was involved in their disappearance was here on the property,” Baker said. “At that point, we locked down the property.”

Police went to the suspect’s home but attempts to contact him were unsuccessful and the tactical vehicle was brought in, Baker said. News video showed that the home was substantially torn open, leaving debris strewn about.

The suspect was arrested about 9.30pm after voluntarily surrendering, Baker said.