The remains have since been sent to the San Bernardino County Coroner’s Office to be examined, police said.
Mr Sparks was arrested on Thursday after “lengthy negotiations” and a search that involved police partially bulldozing his home, where they found a bunker-like concrete structure underneath. Police said Mr Sparks was hiding in it.
Bags of evidence have since been recovered from underneath the home, including human remains, the Redlands Police Department said in a Facebook post on Friday.
Police learned of the couple’s disappearance on 25 August, after the Menards’ car was found unlocked down the road from their home.
The couple’s dog, a small white Shih Tzu named Cuddles, was also missing.
As police were investigating the couple’s disappearance, they said they received a tip to look into Mr Sparks and that he was still on the property.
Police said they tried contacting him over several hours but he did not respond.
His home on the ranch was then breached using a police armoured vehicle, and a search of it was carried out using remote video equipment.
He was arrested later that evening after police found him underneath the home.
At a news conference on Friday, police said Mr Sparks was not cooperative but “surrendered voluntarily” after negotiating with police.
They added they do not have a motive for the alleged killings at this time.
Mr Sparks has been charged with the couple’s murder and was booked at West Valley Detention in Rancho Cucamonga, California.
It’s not clear whether Mr Sparks has a lawyer.
Olive Dell Ranch, where the couple and the suspect lived, is described as a residential RV park and a family-friendly nudist resort, according to its website.
