Orson West, 3, and Orrin West, 4, were reported missing on Dec. 21 in California City, Kern County, about 100 miles north of Los Angeles.
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Jacqueline and Trezell West, their adoptive parents, said the boys vanished from the backyard, possibly roaming into the California desert.
"I saw them there, [I] went in the house, I came back out, I didn't seem them," Trezell West, the boys' adoptive father, told KERO-TV. "I realized that I left the gate opened, and I panicked, came inside the house, searched the house, me and my wife."
Soon, authorities suspected foul play, said California City Police Chief Jon Walker.
"We did a thorough search of the surrounding areas and [the toddlers were] not there, so that tells me that they didn't just keep walking," Walker said. "Someone picked them up."
Police have not named any suspects or persons of interest, but the boys' biological mother blames their adoptive parents.
"They did something. I feel like they did something and they know something," Ryan Dean said.
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The Wests said they understand her anger.
"I would think the same thing ... all I want is to find our babies," Trezell West said.
An investigation into the boys' disappearance remains active, and the reward for information now totals $100,000.
Walker said police requested polygraph tests for "multiple people."
He said that police received multiple tips and leads on the case from the public, but all information is currently being vetted by the department to ensure that it is credible and worth pursuing.
"We haven't given up. We're still trying," he said.