A body that matched the description of Terry Dewayne Smith Jr. was found partially exposed in a shallow grave on the family property, Menifee police Chief John Hill said at a news conference.
The body has not yet been positively identified, Hill said, adding that the death was the result of "a domestic issue" at the house.
Terry has a 16-year-old half brother who authorities have said was the last person to see the boy. However, Hill identified the suspect only as a "family member" and would not confirm if the suspect lived at the house.
Police responded to a tip about the body early Wednesday before they discovered the body, said Hill. Televised news reports showed sheriff's investigators concentrating on what appeared to be a small hole by a tree and about 75 feet from the house, which sits in a rambling, weedy lot off a remote road.
The 16-year-old family member, the boy's mother and other family members were taken in for questioning before the arrest, said Albert Martinez, Riverside County sheriff's deputy.
Missing Since SaturdayTerry had been missing from his Menifee home since Saturday, and hundreds of volunteers have helped search for him.
Volunteers were told to stop searching Tuesday night, because they were getting stranded in the desert and causing problems, hours after local officials pledged to continue searching for the boy.
The boy was reported missing Sunday morning in the Riverside County community about 70 miles southeast of Los Angeles.
Terry was last seen Saturday night, reportedly following his half brother, who told the boy to go home, the Desert Sun newspaper said. The boy's mother, Shawna Smith, said she didn't realize he was missing until the next day.
Searches of the area with a bloodhound, horses and a helicopter were unsuccessful. Hundreds of volunteers also joined the effort but days of covering miles of desert in 100-degree temperatures turned up nothing.
A posting on a Facebook page devoted to the search said Terry's family worried he might have wandered off without food, water or special medication.
However, "foul play has never been ruled out" by investigators, Martinez said.
Searchers Fought Back TearsSeveral dozen people who helped with the search gathered outside a fire station to listen to Wednesday's news conference. Some sat in lawn chairs; others held a small prayer circle.
When word came of a body found matching Terry's description, many began to cry silently and hug each other.
Dawn Berning and her daughters Hannah, 12, and Danielle Stiff, 22, were some of the first residents to volunteer in the search for the boy.
She and her daughters fought back tears after learning a body had been found at the boy's address — and they said their grief was also layered with anger after learning a family member had been arrested.
"I'm devastated," Berning said. "To see something like that happen in our community tears me up. I'm angry. I feel used."